が

Subject marking particle, Identification particle

コーヒーが好すきです。

I like coffee.

Explanation

「が」 has 4 uses

1Saying which one

Points out exactly who or which, not anyone else: 「私がやります」 (I will do it). Questions asking who or which with 誰 or 何 also use が: 「何がいい？」

父ちちが働はたらく。

It's my father who works.

何なにがいい？

What would you like?

2What is happening

Describes what you see or what is happening right now: 「雨が降っている」 (It is raining), 「電車が来た」 (The train is here).

お茶ちゃが冷つめたいです。

The tea is cold.

近所きんじょの子供こどもたちが走はしる。

The neighborhood kids are running.

3Likes, skills, having

What you like, dislike, want, understand or have takes が, not を: 「猫が好きだ」, 「日本語が分かる」, 「お金がある」.

コーヒーが好すきです。

I like coffee.

私わたしは赤あかい車くるまが欲ほしいです。

I want a red car.

4But

After a sentence it means "but", or opens politely: 「すみませんが…」 (Excuse me, but…).

すみませんが、駅えきはどこですか。

Excuse me, where is the station?

この店みせは高たかいですが、おいしいです。

This restaurant is expensive, but the food is good.

Mixed up with

In English you 'like cats' and 'understand Japanese', so cats and Japanese look like objects. In Japanese 「好き」 and 「上手」 are adjectives, and 「分かる」 and 「できる」 describe a state, so their object takes 「が」. Never 「猫を好き」 or 「日本語を分かる」.

が