が / の

Inside a clause that describes a noun, the subject can take の instead of が, and the meaning stays the same. The clause ends in a plain form and sits right before the noun it describes.

Only inside the clause

The swap works only in a clause that describes a noun. In the main sentence, the subject keeps が.

• × 猫の好きです → 猫が好きです

The same goes for とき時, まえ前 and あと後, since they are nouns too.

When to keep が

の reads as "of" whenever it can, so keep が when an object with を comes between the subject and the verb. わたくし私のほん本をかった買ったみせ店 reads as "the shop that bought my book", not "the shop where I bought the book".