My involvement in the online community has driven me to offer grammar lessons to those less fortunate asses that don't know even the basic rules of the one and only language that they speak.
Correction of the day: using "of" where "have" should be. For instance, you *should have* studied more in English class, because "of" is an adverb or prepositon!
No, really. When you contract the words, you don't write "should'f". You write "should've"; the "apostrophe-v-e" is a clue to the word that was contracted. The negative follows the same pattern, did you notice? You wouldn't type "should not of", now would you? The same goes for "would" and "could".
Here's a quick trick: if the phrase is followed by a past tense verb, you would use "have", which is the specific helping verb for imperfect past, and present perfect tenses. Think of it this way: if you had gone out in the rain and were stuck in a restaurant for 13 hours, you might think that you should have stayed home!