Some very common verbs don't follow the +ed rule. Their past form is different — you must memorise them.
go → went
I went to Madrid last week.
have → had
She had dinner at 8.
do → did
We did our homework.
be → was / were
I was tired. They were happy.
There are about 100 common irregular verbs. The good news: most of the very common ones (go, have, do, be, see, get, make...) you'll learn quickly through use.
Most useful irregular verbs (1)
be
→ was / were
have
→ had
do
→ did
go
→ went
come
→ came
see
→ saw
say
→ said (pronounced "sed")
make
→ made
take
→ took
Most useful irregular verbs (2)
get
→ got
give
→ gave
eat
→ ate
drink
→ drank
sleep
→ slept
buy
→ bought
know
→ knew
think
→ thought
read
→ read (same spelling, different pronunciation "red")
Was vs were (to be)
The only irregular verb with TWO past forms
was
I / He / She / It — I was tired. She was here.
were
You / We / They — You were right. They were late.
Negative
wasn't / weren't — I wasn't there. They weren't happy.
Question
Was/Were + subject — Was she there? Were you tired?
Negative and questions: same rules
After didn't and did, use the BASE verb (even for irregular)
Correct
She didn't go.
Did you see it?
They didn't buy it.
Wrong
She didn't went.
→ base verb "go"
Did you saw it?
→ base verb "see"
The past form (went, saw, bought) is ONLY in positive sentences. In negatives and questions: use base form.