The following grammar tips apply whether you are writing copy for marketing, a press release, content for your site, or even if you are a programmer and you place error messages in your code.
Do not underestimate the difference that using correct grammar will make. Although some people do not notice incorrect grammar, many people do, especially those who read a lot (whether it is books, articles or press releases) or for whom writing is a part of their job. If a reader notices that you have used incorrect grammar, you will instantly lose credibility. These tips are for those of you who sometimes have problems with grammar, as some people do. They're listed in order of importance, beginning with the most important.
1. Confusion of "its" and "it's".
I think this is the most common mistake. It is also the most obvious mistake, and the mistake most likely to cause me to lose respect for the writer. When you write the word "its" or "it's", stop immediately and double-check to ensure you are using the correct form of this word. Remember, you should use "it's" when it is short for "it is". Look up the correct usage of these two words, and learn it. If you are still not confident, then you should always ask somebody else which one you should use.
2. Incorrect use of apostrophes in plurals - "browser's" vs "browsers".
This may seem like it is the same error as "it's" above, but in order to understand the correct use of apostrophes you will have to realise that it is completely different. The rules for "its" and "it's" are different to those for other words, so you must learn them separately. Generally, you should never use an apostrophe in a plural - that is, when there is more than one of something. For instance, "bytes", "users", "CDs", "DVDs". Those last two are particularly common - I often see them incorrectly written as "CD's" or "DVD's", which looks terribly unprofessional. On the other hand, it helps me to determine which sites about CDs are respectable and which are not.
You should use an apostrophe when the word you are using is a contraction of two words, for example "this browser's bad" is short for "this browser is bad". The apostrophe stands in for the missing letters when "browser is" is contracted into one word.
In addition, you should use an apostrophe when indicating ownership or possession. For example "Tom's Website" means that Tom owns the Website. "The dog's tail" means that the tail is owned by the dog. Remember that this rule does not apply to "its" because that is a special case. Also, if something is both possessive and plural then the rule is different again (the apostrophe goes after the s). There are more exceptions, and it would pay for you to read a style guide about this.
Do not assume that if you don't know whether to use an apostrophe, then most of your readers won't either. Many of your readers will notice, and they will infer that you did not learn to write correctly. Whenever you write a plural, a contraction, or a possessive noun you should always check to see whether an apostrophe should be used or not, and that if it is to be used, it is in the correct place.
3. Incorrect use of question marks.
A question mark must be used, and must only be used, when the sentence is structured as a question. Take for example the sentence, "We were wondering if you had time to look at the report". This is not a question, and thus it should not have a question mark at the end. It should have a full stop. The following is incorrect: "I wanted to know if you approve of this message?" It is incorrect because the sentence is not a question. For the opposite problem, take the sentence "It is very large, isn't it?" Here, the words "isn't it" are a short form of "is it not" (note the different order). That is a question and must have a question mark.
4. Confusion of "there", "they're" and "their".
This is also an obvious problem to everybody that notices it. However, it is less common than the above problems, probably because it is easier to get this right. So, if you get this wrong, it is very serious. If you know that you have problems with these words (and you would probably know if you did), then you should read more about this and whenever you write any one of them, you should make sure you're using the correct one.
5. Joining words together to form new words.
Do not write "a lot" as "alot". This is not a word. This is sometimes acceptable with words such as "thankyou" and "alright", though in my opinion these are still not ideal.
6. Leaving a personal pronoun out of a sentence.
Personal pronouns are words such as "I", "we", "you", "they" and so on. Sometimes these are necessary, and omitting them can change the meaning of a sentence or cause the sentence to make no sense. Take, for example, the sentence "I don't know what it is". Leaving the "I" out of this sentence will cause it to read "Don't know what it is", which is incorrect and does not make sense logically (it is as if you are ordering me not to know what something is). In verbal conversation, doing this is usually acceptable because the meaning can be inferred from intonation, specifically the emphasis given to words. In writing, this is not acceptable as it forces one to read the sentence twice, with different emphasis, in order to determine its meaning. Keep in mind that the language used in speech and writing is different and has different grammatical rules. Read what you have written at a later time to see if it still makes sense.
7. Unnecessary or inconsistent capitalisation
Do not start a word with a capital letter when there is no reason to do so. Capital letters should be used for proper nouns, such as a person's name or a trademark. They should also be used for acronyms, but only where the letter corresponds to the first letter of some word. I often see sentences much like, "Please consider our Marketing and Development services." This sentence is all over the place. "Marketing" and "development" are common nouns, and you'll find them in the dictionary. They should not have capital letters.
When you are writing the title of something, for example "The Shawshank Redemption" or "Build Your Own Database-Driven Website Using PHP & MySQL", you should capitalise every word. For common words such as "a", "the", "of", you may choose not to capitalise these if they are not at the start of the title. Note that if the title is actually a sentence, with a full stop or similar, then it should not be capitalised as if it is a title. For instance "Place your order now" is a sentence, not a title.
Side note: using a capital "W" for "Web" is acceptable because "Web" in this case does not describe a web as defined in the dictionary, but it is a trade mark created by Tim Berners-Lee (part of "World Wide Web"). However, using a capital "I" for "internet" is not recommended as "internet" is usually used as a common noun.
Many other grammatical problems exist, but I have identified the most common and the most obvious. There are some "grammatical rules", not listed above, which are open to opinion and may be broken. Something may use correct grammar and still be awkward to read or understand. Only an experienced writer will know what to do in those situations. However, the above seven points are highly important, and crucial. If you can fix all of the above seven problems in your writing, then you will come across as more professional. If writing is a part of your job, and you cannot write using correct grammar, it is very important that you learn to do so.
I see the above common mistakes on a lot of websites, even in error messages. If you are a programmer, you are not exempt from the need to use correct grammar. The error or status messages you write will be seen by the public, and an error message with poor grammar will reflect badly upon your product at the very time that the user has encountered a problem.
We all have weaknesses in our writing style. Try to identify yours, but try and focus specifically on the above seven points. We also all make simple typos, and these can often be forgiven, though you should proof-read what you have written.
Note: This list originally had six points, but I added the seventh afterwards. I also expect at least one person will point out the errors I made in this post