what is simple past tense

Past tense means the things that have already past

example:I went to school yesterday.
Because "yesterday" so it is a past tense

example word:a month ago,yesterday,a year ago,this morning,yesterday night

One more example: I did my homework this morning.
Because this morning so it is a past tense

2012-02-29 20:27:01 補充：
Simple past word example:
sleep>slept
can>could
may>might
study>studied

In normal way,the word will add ed,like played
while end with y change to ied,like studied
The word before the last word is vowel,double the last word like fitted

2012-02-29 20:27:55 補充：
Some word like buy will change to bought

2012-02-29 21:09:47 補充：
The simple past tense MUST use in verb

It means that the verb is usually used +ed in general.
But sometimes it has exceptional. For example:
the verb as for buy and its past tense is buy,bought,bought.
And the simple past tense is used in describing the event happened
before.Take an example :I went to see the movies last night.