how to say “amazing” in Hebrew
The Hebrew word for amazing is מַדְהִים ( ). This is an adjective deriving from the active-intensive הִפְעִיל verb, לְהַדְהִים ( ).
For example:

I saw an amazing movie yesterday.
and, in the feminine:
What amazing music!
The root is ד.ה.מ meaning amazement or being taken aback. Another usage (the one in ירמיהו יד, ט – Jeremiah 14:9) is amazed – נִדְהָם ( ) in the masculine and נִדְהֶמֶת ( ) in the feminine. These words invoke the “passive”-simple נִפְעַל verb form. They also function as verbs, for example:
She was amazed at (by) the sight.
Synonyms for amazing include:
מַטְרִיף – (also) crazy-making
מְהַמֵּם – stunning
…and others.