How to express "the" in Chinese
The definite article has no equivalence in Chinese. Its functions, however, can be found in Chinese.


Something previously mentioned or known


Compare:

He is a murderer. 他是个杀人犯。(杀人犯 is a murderer, a man that has commited murder. 个 is like the indefinite article.)

He is the murderer.

By using nouns inherently specific.

他就是凶手。(凶手 is the murderer , it cannot be used without context.
他是个凶手(incorrect))

By using demonstratives: 这/那/这些/那些/此/彼

他就是这个杀人犯。


Something unique or identified by the speaker, terminologies.


I cannot pronounce the four tones well.

我发不好四声。(平上去入(tradition)/阴阳上去(modern mandarin), as a whole isn't referred as 四个声调)

Particularly, these sets of similar things are usually referred without measure words.

四书/十三经/六艺/五行/四君子/十八罗汉/一百零八星宿......

Unique things, such as the sun, the moon... are referred exactly its name (Actually the definite article is unnecessary semantically)

As for "I want to go to a/the park", it's not hard to appreciate that 我想去公园。 already fully conveys the two possible interpretation, depending on the listener.