The three principal parts of verbs are the present tense, the past tense, and the past participle. (The present participle or "-ing" form is sometimes considered a fourth principal tense.)

:: Present Tense
Present tense indicates an action in the present:

Now the class begins.

She walks to class.
 

:: Past Tense
Past tense indicates an action that occurred in the past:

We wanted to see the show.

The little girl blew a bubble.
 

:: Past Participle
The past participle can be used as an adjective or modifier. It is typically formed by adding 'd' or 'ed' to the base form. Many times, this form is identical to the past tense of the verb:

Since the dishes were washed, we left the kitchen.

The broken vase sat unceremoniously on the kitchen table.
 

:: Common Mistakes
There are many irregular verbs (about 250) that confuse writers when forming past tense and past participles. Here is a sample of irregular verbs.