Let's talk metronomes! What are they? Are they important? Why are they so annoying?
First, definition: a device used by musicians that marks time at a selected rate by giving a regular tick(Oxford Dictionary). In practice, it's that little ticking device that your teacher probably turns on every lesson that just causes more confusion. You can choose a specific tempo, and the metronome will tick away at that tempo until the end of eternity. It will not slow down or speed up unless you change the tempo.
The question every student asks, of course, is "Why can't I just keep tempo in my head?" The short answer is that you can't. No, really, you can't. Don't trust the tempo in your head. Keeping regular time is a learned skill, and truthfully, I'm not quite sure you can ever just totally trust your mind to keep an accurate tempo. It's easy to slow down at those difficult parts and speed up at the easy ones and just not even know that it's happening.
I'm sure your next question is, "Why can't I count out loud?" Well, you could, and it would probably be more accurate than doing it silently. In fact, I'm sure that's what musicians did before the metronome was invented. (It was in 1815 in case you're wondering. I just looked it up.) But the truth is, even counting out loud can lead to faulty timing. The best and most accurate way to keep time is with a metronome.
Ok, but let's talk about the wonderful things you can do with a metronome! A metronome can help you with those impossible measures full of strange timing. Simply mark a line through the measure at each beat, and use a metronome to help you know where in each beat each note would fall.
Another incredibly useful way to use a metronome is to check your consistency. Once a piece has been learned, it's great to go back and play through the piece with a metronome to make sure that you are playing the entire piece at the correct pace.
Now, one thing the metronome does not account for is the fluidity of music when it comes to emotion or mood. This is mostly for intermediate players and on, but when bringing emotion or expression into a piece, you may naturally change tempo temporarily. So, a measure may be very intense and may be drawn out, or it may be quickened a bit (Chopin is a big fan of this.) Either way, your metronome is not going to change with you. So, at some point, the metronome must be put away, and you must play the piece with natural expression. Of course, don't take liberties with a piece to the point it is unrecognizable or difficult to follow. Musical expression is a learned skill that is extremely fun to practice!
Using a metronome can at first be daunting and a bit frustrating. But keep in mind that it is an incredibly useful tool. Happy practicing!