Particles are probably one of the most confusing things about learning Japanese. Due to the fact that particles all have their rules and uses, and some verbs using certain particles, among other things, students often find themselves using the wrong particles or forgetting which particle to use.
So we'll start out with the particle は . In the previous lessons, we used は in the sentence "X はY です。" は is pronounced "wa" and not "ha," even though it uses that character. What is は? It is a topic marker. It is used to mark the item of focus in a sentence.
せんせいはわかいです。(sensei wa wakai desu)
The teacher is young.
In this example, the subject, せんせい, is the item of focus. Keep in mind though that は marks the item of focus and not necessarily the subject of the sentence. Take for example this sentence:
せんせい、しゅうまつはいそがしいですか。 (sensei, shuumatsu wa isogashii desu ka?)
Teacher, is your weekend busy?
As you can see, the subject of the sentence is せんせい, but the object of focus within the sentence is しゅうまつ (weekend). You'll often find it marking other things besides the subject, which is why it is important to remember that it only marks the item of focus in a sentence.
Of course though, this can also help you greatly. Since you know that は follows the object of focus, you'll know what it is. It can come in handy when someone asks you a seemingly convoluted question:
せんせいがわたしにくれたしゅくだいはどこですか。 (sensei ga watashi ni kureta shukudai wa doko desu ka?)
Where is the homework that the teacher gave me?
I know I've included grammar that is a little more advanced and particles that I haven't mentioned yet, but the point is that when you initially look at a sentence like that, it seems like a lot of things are being said. However, just focus on は. The noun before は is しゅくだい (shukudai = homework). Even though しゅくだい is modified (せんせいがわたしにくれた = given to me by the teacher), は marks the item of interest, which is simply just しゅくだい.
Without even knowing much else of the sentence, you're at least able to realize that they are asking where their homework is. Now, you can point to somewhere near them and yell out "そこです！” (soko desu = there).