What are those words that change the entire spelling when it is in past tense?

You're asking about irregular verbs, which don't follow the standard "add -ed" rule for forming the past tense. Here are a few examples:

Present Tense | Past Tense

------- | --------

go | went

see | saw

eat | ate

take | took

have | had

do | did

say | said

make | made

give | gave

come | came

There are many more irregular verbs in English, and they can be tricky to learn. A good way to get familiar with them is to practice using them in sentences and look at lists of common irregular verbs.