In affimative sentences the verb gets - ED or you use the 2nd form of the irregular wordlist (you can find this in you workbook on page 136
Example: I studied for my test
I had a great mark for my test
In negations you use DIDN'T + VERB in present tense
example: I didn't study for my test
In questions you use DID + VERB in present tense
example: Did I study for my test?