Clauses are, literally, the stuff that sentences are made of. The briefest expression of a full thought. More complex sentences might be composed of multiple clauses, but some clauses are full sentences in and of themselves.
If that gives you pause - but wait, I thought that words were the stuff that sentences were made of? - then don’t worry, you aren’t wrong. Let me phrase things a bit differently.
If I asked you to describe your hand, most people would probably talk about palms and fingers. That being said, your hand is actually composed of twenty seven individual bones. Very few people would just list all those twenty seven bones out, however, and that’s because the things these bones come together to make are more useful, easier to recognize and grab onto, than the individual bones themselves.
A finger is the smallest thing a hand can be broken down to that is still useful and a clause is the smallest thing a sentence can be broken into that still communicates a full thought. A clause could indeed be broken still further down into the words it is in turn made of, but unlike a clause, these words aren’t things you’d normally utter in isolation. Just like a knuckle is part of a finger, a single word like beautiful or even a few words like softly snoring dog are merely parts of a clause.
Main clauses, also known as independent clauses, form the integral core of any given sentence. Japanese is a language organized around verbs: a sentence can consist of only a single verb. This is not because a subject genuinely doesn’t exist, rather that Japanese commonly omits information that is made obvious by context. If there is more than one word in a sentence, the sentence will end in a verb. The order of everything else leading up to the verb is quite flexible.
(私は）知らない。( I ) don’t know.
(私は）メロンパンが好きだ。I like melon bread.
(私は）ソウルで初めて彼と会いました。I first met him in Seoul.
Subordinate clauses, also known as dependent clauses, provide information about the main clause. They are not complete sentences and thus “depend” on the main clause - they can never exist alone. There are a few different types of subordinate clauses: some function as nouns, others as adjectives and others as adverbs.
合格したかどうか知らない。I don’t know whether I passed or not.
彼女はロシア人だと信じる。I believe that she is Russian.
ラメんが温かいうちに食べてください。Please eat the ramen while it’s still hot.
When I went back (to my home country), everybody came to greet me at the airport.
Relative clauses are a type of subordinate/dependent clause which function like adjectives: their purpose is to describe a noun. We usually introduce relative clauses with the relative pronouns who/that/which/whose/where/when in English, but Japanese relative clauses can immediately proceed the words they describe.
しゃべる山羊は見たことがない。I have never seen a goat that talks.
Yesterday, I went to to watch a movie that I wanted to see with a friend.
I hope to meet someone who likes melon bread as much as me someday.