We say: go to bed/be in bed/ etc. (not the bed)
It’s time to go to bed now. This morning I had breakfast in bed.
But: I sat down on the bed. (A particular piece of furniture)
We say: go to sea/be at sea (without the) when the meaning is go/be on a voyage
Keith is a seaman. He spends most of his life at sea.
But: I’d like to live near the sea. It can be dangerous to swim in the sea.
When we are talking about things or people in general, we do not use the.
I’m afraid of dogs.
We use the for music instruments:
Can you play the guitar?
The piano is my favorite instrument.
We use the with some nationality adj to mean ’the people of that country)
The French are famous for their food. (=the people of France)
We use the in names with ‘republic,’ ‘kingdom’, ‘states’
The United States of America (USA) the United Kingdom (UK)
The Dominican Republic
We do not use the with names of most streets/roads/squares/parks ect.
Union street (not the…) fifth Avenue Hyde Park blackrock Road