What are the correct forms for past simple, past perfect simple and past perfect continuous tenses?

Great question! English verb tenses are notoriously difficult to master. Here's a chart I made up to help you:

Past

Present

Future

Simple

I ate pizza.

I eat pizza.

I will eat pizza.

Continuous

I was eating pizza.

I am eating pizza.

I will be eating pizza.

 Past Perfect

I had eaten pizza.

I have eaten pizza.

I will have eaten pizza.

Past Perfect Continuous

I had been eating pizza.

I have been eating pizza.

I will have been eating pizza.

Past Simple

As you can see, simple past is the most basic form of past tense. Usually in English, the past ends in -ed, like "cooked" and "helped." But just to make life more difficult, many verbs in English have irregular past tense forms like "ate" and "ran." These just have to be memorized.

Past Perfect Simple

The past perfect simple is simply "had" + the past simple. 

E.g. I had discovered uranium!

E.g. I had loved the book!

The past perfect is used to communicate an action that is more past than another action, like in this example:

After we had eaten dinner, we went out for ice cream.

Both actions happened in the past, but the dinner happened first, so it's in the past perfect.

Past Perfect Continuous

The past perfect continuous is "had been" + the -ing form of the verb.

E.g. I had been eating!

E.g. I had been swimming.

Like the Past Perfect Simple, it is more past than another past event, but in this case it's continuous, i.e. going on for a long time.

Good luck with your study of English grammar, I know that it's very challenging!