前の人の頭がじゃまでステージがよく見えなかった。って英語でなんて言うの？

Yes, sometimes your view may be blocked because someone in front of you is rather tall, or has a long back. There is nothing to be done in this situation other than change seats if possible. You may explain after the show:I couldn't see very well because of the person in front of me."

When something is "in the way" it means that it is making a hindrance or obstacle to a movement or action. In our case, the man's head was "in the way", so his head was being an obstacle to a clear view of the show.

Sentence examples: I could not see a thing on the stage with his head in the way! It was so hard to watch the performance with that man's head blocking my view. Tall people like him should be made to sit at the back. He is in my way and blocking off the stage completely.

If the seats of a theater are not arranged in such a way that people do not block each other, it happens that if the person sitting in front of you may block your view if he has a big head. In such a case, you may not enjoy the show because of not having a good view. So, after the show, you may say: His head was blocking my view and I could not properly see the show. or His head was right in the way and I couldn't see the show. or My view was blocked by his head and I couldn't enjoy the show.

You can use the phrase "blocking" when talking something being in your way. "A person's head was blocking my view of the stage/show."

When you are sitting trying to watch something the person in front of you might get in the way and you can't see as well this is also called 'blocking your view' To explain someones head was in the way and you couldn't see well then you can simply say 'his/her head was in the way!'

I wasn't able to see them, as the guy in-front of me was blocking my view. - this just explains why you were not able to see the performers on stage. The guy in front of me was blocking my view, so I could not see them very well. - this one does the same thing, just provides a different wording of the scenario.

When someone "blocks you view" they are between you and what you want to see. This is the same as an "obstructed view". This means you view is blocked by an obstruction (the head of the person in front of you). If something is "hard to see", you can't see it easily. This is the same as "difficult to see" but is a bit more casual.