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I wish i'd known. past perfect i think ?
You should go to the doctor. present continuous i think ?
I'll visit my grandmother tomorrow. future perfect ?
I'm going to visit my grandmother tomorrow. present continuous ?
They stopped to talk to each other. present continuous?
They stopped talking to each other. present perfect ? i think

And if possible an explanation of why i may be right or wrong. So i can learn from my mistakes.
thankyou.
Comments  
I wish I'd known. This sentence is INCORRECT. The correct form would be, for example:

I wish I knew him.
I wished I had known him.

You should go to the doctor. In this case you don't have a verb tense. You have a modality because of the modal verb SHOULD. However, the idea is of present.

I'll visit my grandmother tomorrow. FUTURE.

The future perfect would be like: When you return I will have visited my grandmother.

I'm going to visit my grandmother tomorrow. FUTURE with "GOING TO"

They stopped to talk to each other. SIMPLE PAST.

They stopped talking to each other. SIMPLE PAST.

Pay attention to the last two sentences, the idea in each one is different.

In the first one: "They stopped to talk to each other" you have the idea that maybe they were walking on the street then they met and stopped to talk.

In the second one: "They stopped talking to each other". You have an idea that maybe they had a fight and they don't talk to each other anymore.
Thanks!
You helped me a lot! xx