刚才 (a moment ago)

Thirty seconds ago is already history

• just now (variant of 刚才)刚才gāng cái names a tiny slice of the recent past: "just now / a moment ago".
• It's a time-word, so it sits where yesterday昨天zuó tiān would.
• just now (variant of 刚才)刚才gāng cái他来tā lái(completed / change)了le = "he came a moment ago."

just now (variant of 刚才)刚才gāng cái + Clause

• Put just now (variant of 刚才)刚才gāng cái at the front, or right after the subject.

a moment ago刚才gāng cáihe他tācome来lái了le。

• just now (variant of 刚才)刚才gāng cái他来tā lái(completed / change)了le = "he came a moment ago."

It points at the past

• just now (variant of 刚才)刚才gāng cái pairs with things that already happened — often with (completed / change)了le.

you你nǐa moment ago刚才gāng cáisay说shuōwhat什么shén me？

• you你nǐjust now (variant of 刚才)刚才gāng cáivariant of 说说shuōwhat什么shén me？ = "what did you say just now?"

Recap

Read the full reference: HSK 3 grammar — Nouns.