是 can be used as all forms of “to be", but only when equating subjects to other nouns. In order to connect subjects to adjectives, you would use 很.
The most basic function of 是 is to equate a subject with another noun.
The 是 . . . 的 construction can be used in many situations, such as . . .
When you want to emphasize one piece of information:
When you want to omit an already obvious noun:
When you want to talk about what someone does for work:
When you want to explain someone’s purpose by adding 來:
Or something’s purpose by adding 用來:
Furthermore there are a few adjectives that can only be connected to the noun with the 是 . . . 的 structure, rather than the more commonly used 很:
In these examples you can only use 是 . . . 的 because there aren’t really varying degrees of being biologically male or being Sichuanese.
While the 是 . . . 的 pattern can be used to emphasize one part of a sentence, just using 是 can be used to emphasize the entire sentence.
是 can be used to point out that only one kind of thing exists in a certain location. To simply indicate the existence of something in a location, whether its among other things or not, use 有.