Past simple - 1

With regular verbs you make the simple past by adding '-ed'.
Unfortunately, there are a number of irregular verbs and with those, this rule doesn't apply.
The irregular verbs have their own form that you have to memorize/study.

present tense	past tense
to be	was/were
to buy	bought
to come	came
to do	did
to drink	drank
to eat	ate
to feel	felt
to find	found
to get	got
to give	gave
to go	went
to know	knew
to leave	left
to make	made
to meet	met
to read	read
to say	said
to see	saw
to sit	sat
to sleep	slept
to take	took
to teach	taught
to tell	told
to think	thought
to understand	understood
to wear	wore
to write	wrote

You can find various lists of irregular verbs on the internet.