How does Shakespeare make Macbeth's "Is this a dagger which I see before me / The handle toward my hand?" soliloquy a dramatically effective moment?
The "Is this a dagger which I see before me" soliloquy in Shakespeare's Macbeth is dramatically effective for many reasons, but I'd like to focus on Shakespeare's use of apostrophe. As a recap, an apostrophe (not to be confused with the punctuation point) is an address, either to a character who isn't present, or to an abstract idea or personified object. Shakespeare uses apostrophe to great effect in many of his plays, and his use of it in Macbeth's soliloquy in Act 2, Scene 1 is one of the best examples of the literary device (and also my personal favorite).
Macbeth uses an apostrophe to address the hallucination of a dagger. Since this soliloquy occurs directly before he murders King Duncan, it's a very dramatically effective moment. For example, the hallucination of a knife comes to personify Macbeth's murderous ambition. Moreover, it proves that Macbeth is beginning to become unhinged and is preparing to slide into insanity. As such, the apostrophe in this soliloquy creates ominous foreshadowing that hints at the violent deeds to happen off-stage, and it also gives us an insight into Macbeth's personal degeneration. As such, the soliloquy as a whole becomes a tense moment upheld by robust poetry.
Because the soliloquy is a long one, there are many other ways that it is dramatically effective, and I'd encourage you to explore those different possible ways for yourself. However, for me, the soliloquy's apostrophe to the imagined dagger is its most dramatically effective moment.