When is the used or a/ an? The first choice is between specific or general.
The word teacher is general in the sentence:
He is a teacher.
If we add information that separates that teacher from all the others then we use the.
He is the teacher who taught me in Primary Three.
Sometimes neither the or a/ an is used.
"He is my Primary Three English teacher." (Neither the or a/ an is used. Instead my is used to put him in context.)
The is used for specific uncountable things. Uncountable things like water do not use the unless they are specific.
He spilled the water, from my bottle, on the floor.
He spilled water on the floor.
A/ an only gets used with things you can count.
The second choice is for the names of organisations and places.
Sometimes the is used with place names and sometimes it is not. The differences are small and usually don't interfere with communication. There are lists of rules for different types of places. Here is a simplified version. Use the only for place names with specific descriptions that answer the question which.
Names of rivers, deserts, forests, seas, bays.... all use the type of place in the name.
Which river is it? The Yangtze River
Which desert is it? The Gobi Desert
Country names with descriptions use the. Which group of states is it?
The United States
The People's Republic of China
But the is not used for America or China without the description united or people's republic.
Then there are the unusual uses.
Usually groups of islands or mountains have a the, but not single islands or mountains.
The Philippines, The Rocky Mountains
Hong Kong Island, Mount Everest
Specific abstract geographical terms use the, like the North Pole and the Equator.
The Middle East
As usual in English there are exceptions. The group of islands called Indonesia should use the, but doesn't. Some people argue about whether or not Ukraine should use the. In cases like this just Google it. Compare "in the Indonesia" and "in Indonesia". Pick the more popular option, use it and don't worry about it. If you are not sure, guess, and at most you will sound awkward. It is better to try and be awkward than to give up.