Verb (fi'l)

The fi'l shares half its definition with the ism: both mean something in themselves. The whole difference is time. Say قَلَم and nothing has happened; say كَتَبَ and something happened, and it happened before now.

Arabic has exactly three verb forms, one for each time:

الْمَاضِي — what happened before the moment of speaking: كَتَبَ، فَهِمَ، دَخَلَ. Its sign is that the silent feminine tā' can join it — كَتَبَتْ.

الْمُضَارِع — what is happening now or will happen: يَكْتُبُ، يَقْرَأُ. Its sign is that لَمْ, سَـ or سَوْفَ can come before it. It always begins with one of the four letters gathered in أَنَيْتُ — hamza, nūn, yā', tā'.

الْأَمْر — what is being asked for: اُكْتُبْ، اِجْلِسْ. Its sign is two things together: it asks for something, and it accepts the feminine yā' — اُكْتُبِي.

A word can name an action and still not be a fi'l. الْكِتَابَة (“writing”) names the act but carries no time, so it is an ism — a masdar. Time is the test, not action.

Does Arabic have a future tense?

Not as a separate form. The mudāri' covers both present and future, and a particle decides which: الْآنَ or context for the present, سَـ or سَوْفَ for the future.

Is صَهْ (“be quiet”) an imperative verb?

No. It asks for something but will not accept the feminine yā', so it fails half the test. Khālid classes it as an ism fi'l — a noun standing in for a command.