speak English

NewguestHi

Are both fine?

1. You have/must to speak English with him.

2. You have/must to talk English to him.

You have to speak English with him.

How can I say the same thing using the word "talk"?

You have to talk English with him.

With specific languages, the more idiomatic word is speak, so it's not very often that you will hear talk English, talk Spanish, talk Chinese. These are normally said as speak English, speak Spanish, speak Chinese.

CJ

I know that they say "I'll talk to him" So, can I also say "You have to talk English to him"

Yes. They're all OK.

You have to [speak / talk] English [with / to] him.

But speak is much more usual with the names of languages, as mentioned above.

CJ

5. I can speak English.

6.I can speak in English.

I don't know the difference in the meaning between the above two. I know they are fine.
Please tell me.

Both are fine. The first one (numbered 5.) is more usual.

CJ