Function 1: the noun before が is the subject
He is Tanaka.
“He” is the subject
Function 2: express the hope
I want to know the result.
“The result” is something you want to know，It is your hope.
Function 3: express likes and dislikes
I like sweet food.
“sweet food” is something you like.
Function 4: express the level of skill
She is good at painting.
“be good at” is the level of skill.
Function 5: express the difficulty
This pen is hard to use.
“hard to use” is the difficulty.
【Polite Form of verb（delete the ます in the end）＋にくい 】 means “hard to”. It’s a grammar rule. For example, 食べにくい=hard to eat, やりにくい=hard to do.
Function 6: express the ability
I can read kanji.
“can read” is the ability.
は is also a particle（adverbial particle）. When は is a particle, it reads as “wa”,
Remember that は emphasizes the noun after it. But が emphasizes the noun before it.
In addition to this function, は has another function: contrast.
This building is tall, compared to that, that building is not tall.
I’m Japanese.（は emphasizes the noun after it，which is “Japanese”.）
I’m Japanese.（が emphasizes the noun before it，which is “I”.）