We reduce “function words.” These types of words generally don’t carry as much importance or meaning as the content words. If they were eliminated, the sentence would still make sense. Here is a list of the function words:

a. pronouns - he, she, you, they, mine, his, himself, etc.
b. prepositions - to, in, for, at, by, on, with, from, etc.
c. conjunctions - and, but, or, nor, so, yet
d. auxiliary verbs - am, is, was, were, do, does, been, have, can, could, should, etc.
e. articles – a, an, the
f. indefinite pronouns - one, some, any, anywhere, somewhere, anything, something, etc.

There is one exception to the rule above: auxiliary verbs are stressed in their negative forms. See below.