Write what you know. That's what they teach you in school, but in the world of professional copywriting you'll often be called upon to write content on subjects that are foreign to you.
How many times have you seen phrases like "we can offer you", "we will develop" or "we have created" on a web site?
Verbs like can, will, and have take your web copy out of the present and either put it in the future or the past. Using future tense and past-tense phrases is one of the biggest mistakes copywriters make in writing web copy.