Important irregular verbs:

Be
I am
She/He/it is
We are
They are

Have
I have
She/he/it has
We have
They have

Can
I can
She/he/it can
We can
They can

Do
I do
She/he/it does
We do
They do

Past Simple (Regular):
For most verbs, add -ed, or -d
e.g.
Walk ⇒ Walked
Ask ⇒ Asked
Need ⇒ Needed

CAREFUL!
(if the verb ends in consonant + y), change the ‘y’ to ‘i’ and then add -ed.
e.g.
Study ⇒ Studied
Cry ⇒ Cried
Try ⇒ Tried

Negative
Present simple
Subject + auxiliary + not (‘nt) + verb
e.g.
I can’t speak French.
I don’t like cheese.

Past Simple
Subject + auxiliary in the past + not (didn’t) + verb
I couldn’t speak the language.
I didn’t like the food.

First/Third person
I speak English ⇒ He/she speakS English.
I do the washing up ⇒ He/she doES the washing up.