"There is no homework in this class."
The trick I use is think about which noun is being attached to the verb. In this sentence "isn't" is the verb and the noun attached to it is "homework." Therefore, class will get は and homework will get が.
I have also been told that が can be thought of as emphasizing something in a sentence in certain cases aka highlighting the subject. While true the sentence is ultimately talking about a class, the verb indicates you are talking about the homework.
Hope this helps!
Maybe? I’m not actually sure if Japanese is ok with a double subject, but my gut feeling leans towards it being ok. However it would probably be more along the lines of “THIS class has no homework“ (as in the answer to “which class has no homework?”). I would have translated “This is the class with no homework” as しゅくだいがないじゅぎようはこれです. ”concerning the class without homework, it is this one.”
Basically the thing you have to get used to is that Japanese makes a very clear differentiation between what is the topic of conversation (the thing that you are talking about) and what is usually referred to as “comment” (something that is said about the topic).
Typically the topic is something that is already known, while the comment is some new information about it. So if for example I say: 日本語はおもしろいです “Japanese is interesting”, I presuppose that Japanese is something you know of and then give you more information about it.
But maybe your question was not “what do you think about Japanese” but rather “what is interesting?” In this case, “Japanese” would be the new information, which means it is not the topic, so I can’t use は. Instead I default back to the subject marker が because 日本語 happens to be the subject of the adjective おもしろい:
- 何がおもしろい？ “What is interesting?”
- 日本語がおもしろい。 “Japanese is interesting.”
When you see が, it always marks the subject of the verb, and that subject is part of the new information about some topic of conversation rather than the topic of conversation itself. However you should be careful because the subject in the Japanese sentence might not be the subject in the most natural English translation because the two languages may use different ways to express the same fact. For example in our sentence above:
このじゅぎょうはしゅくだいがありません. “This class has no homework.”
At first glance you might think that が marks the object of “to have” here. But this is because Japanese phrases possession differently. The Japanese sentence is literally: “As for this class, there is no homework.” Where English has a dedicated verb “to have” with a subject and an object to say “A has B”, Japanese basically says “as for A, B exists”, with A as the topic of the sentence (the one that we give new information about) and B as the subject of the existence verb ある/いる (depending on whether B is animate or not).
And in those cases where the topic also happens to be the subject of the verb at the same time, you only use は, no が:
わたしはやさいをたべます。 “I eat vegetables.”
“I” is both topic and subject at the same time, so you use only は and not が.
Does that make things a little clearer?