Can I re-teach Thief to my Pokemon in Emerald?

Question

I decided to forget thief in order to learn a new move. are you ever able to get TM moves ever again?

Answer 1

Some TMs can be repurchased, but Thief isn't one of them. You can never get another Thief TM in your Emerald save without trading.

TM046 (Bulbapedia)
Oceanic Museum (Bulbapedia)

Hope I helped!