Translation:I like bicycles.
は marks the topic. This can be the subject or direct object or anything else. In case of the subject and the direct object, the original particle が or を is left out. It is important to understand how the topic works. The topic introduces about what you speak in general, about what you provide information. Also, the topic is left out often, the same way we use pronouns instead of the actual word. In the above example, the topic is "I" => you provide information about yourself. But the topic is left out. The complete (but weird) sentence would be 私は自転車が好きです, translation is something like "Bikes are liked by me". You should also note that 好き is not a verb in Japanese. It is more like an adjective, for example you can say "my liked bike" 好きな自転車 and therefore, the closer translation of this Japanese sentence would be "Bikes are liked". This explains why, when it's not the topic (because the "by me" is the topic), the particle が is used here. Now how do you get what is the topic? The topic is generally more what you provide information about, not the information provided itself. It is often introduced once (e.g. by saying a person's name and the は particle) and then left out until the topic of the conversation shifts. Also, the topic I/me/myself is often left out completely. You can also make "today" the topic etc. And whenever it's not the topic, you use the particles が for subject or を for direct object. Let's take an example. First, a weird sentence without a topic: 私がパンを食べます If you are talking about yourself, and the information you want to provide is "eating bread", you would write （私は）パンを食べます ((As for me, I) eat bread) But if you are talking about bread, and the Information you want to provide is "I eat it", you would write パンは私が食べます (As for bread, I eat it) Note that the topic, if provided, often comes before the subject, although Japanese is more flexible than English in terms of word order because in English the role of a word (subject or object) is determined by the position of the word, but in Japanese by the particle after the word. So in Englisch, when you take the sentence The cat eats the mouse. And you change the word order into The mouse eats the cat. you got a different meaning. In Japanese, you would have to change the particles for that, just swapping the words for cat and mouse would not have any effect on the meaning. So in the above sentence, you might also write 私がパンは食べます This seems a bit unusual but it's still correct grammar.
Now imagine a vegetarian talking about not eating meat. What particle would be used in a situation, 1) when the person is introduced or 2) when you are talking about what to eat for lunch/dinner.
It is indeed "wa" for topic and "ga" for subject.
Topic is not necessarily the subject of the sentence. Sometimes "wa" can be used with objects as well. It's the main theme of the sentence.
Many people try to show the role of "wa" with sentences like:
- As for bikes, I like them
Many times, using "ga" can be the same as using "the".
Japanese enables what English speakers would consider incomplete sentences, which can sometimes help you figure out whether or not you should use は or が.
For example, I could ask 君は質問がありませんか。(kimi/you)(wa/topic)(shitsumon/question)(ga/subject)(arimasen/exists)(ka/question). “Do you have a question?”
But it’s a question, and I’m asking someone else, so I could legitimately leave out the topic, because if I’m posing it without a topic, it would suggest that I either a) do not know my own mind about whether or not a question exists or b) I’m asking the question of someone else: 質問がありませんか。(しつもん/question)(が/subject)(ありません/does not exist)(か/question). Literally “Does a question not exist.”
The fun bit? When I ask that of someone at work, they’ve answered: あるよ。Literally “is” or “exists,” the opposite of my negative verb. I know a question, with regard to them, exists. They have a question. Japanese can thrive on incompleteness and inference.
So let’s go back to this exercise: じてん車が好きです。
It’s not a question; there’s no か at the end, so the speaker is either a) telling you you like bicycles or b) telling you they like bicycles. Either way, the topic is assessed by context; the speaker is most likely telling you they like bicycles. You can fill in the topic: 僕はじてん車が好きです。
In this case, Duolingo has a bit of a problem. が marks the subject, unless it is also the topic and therefore needs は. Finding the subject in an isolated sentence is generally easy (find the verb, then ask "who/what <verb>?" and the answer is the subject), but to identify the topic you need to know what has been said before. Many, if not most, Japanese sentences are still fully grammatical if が is changed to は (but note that the other way around doesn't necessarily work, since は can mark objects, adverbials and other constituents just as well as the subject, if they happen to be the topic), so there is no way to create a simple rule that works on a sentence-to-sentence basis.
There are patterns -- for example, います and あります are generally used to introduce new things into the discussion, which then by definition can't be topic, and therefore tend to use が -- but for the most part this is just something that is very hard to learn through Duolingo, but once you start reading actual text it should be a lot easier.
Sometimes there’s a か゚ pronunciation instead of a が, like the speaker has more of a Tokyo accent. Depending on the syllable leading into it, it can sound like a は.
I have heard similar effects in other exercises with other particles. For example, sometimes the を in a sentence practically disappears because of the speed and the accent.
Strictly speaking, yes, but if, in context, you had been talking about your bicycle, it would be OK. “じてん車が好きです” has no topic, the subject is a bicycle, not a bike (which would be バイク), and there’s no possessive (僕の).
Lacking a topic, it’s implied, probably the speaker (yourself). Lacking a possessive, it’s just bicycle(s).
Hence, “I like bicycles.”