友だちが家に来ます 👉 A friend is coming to my house tomorrow
が follows the subject of a sentence with a verb.
空がくらいです 👉 The sky is dark
が follows the subject of a sentence with an adjective.
兄はゴルフが好きです 👉 My older brother likes golf
Use が before 好き、嫌い and ほしい.
この漢字の意味がわかりませんか 👉 Do you understand the meaning of this kanji?
Use が before できる and わかる
これは私が撮った写真です 👉 This is a photograph taken by me
In a noun modifying clause は is replaced by が.