Learn Hardcore Japanese: haru ha ame no hi ga ooi desu. - There are many rainy days in spring.

は is the topic marker. It tells us that 春 (“spring”) is the topic we’re talking about. A natural English rendering is “As for spring,…” or “When it comes to spring,…”.

Why is there a の between 雨 and 日?

の is the genitive (or noun-linking) particle. 雨の日 literally means “day of rain,” i.e. a “rainy day.” It creates a single noun phrase from two nouns.

Why is 日 followed by が instead of another particle like は?

が marks 雨の日 as the grammatical subject whose quantity is being described by 多い. You use は to set a topic, but が to indicate the specific subject that has many instances.

Why do we use the adjective 多い rather than a verb form like 多くある?

多い is an i-adjective meaning “many” or “numerous.” It’s the most natural way to say “there are many X.” While you can say 雨の日が多くあります, this construction is more wordy and formal. 多いです is more colloquial and common.

What’s the function of です at the end?

です is the polite copula (or polite ending). It doesn’t add new lexical content here but makes the statement polite. Without it, 春は雨の日が多い would sound more casual.

Why not just say 春は雨が多いです?

Saying 雨が多い would literally mean “there is a lot of rain,” but Japanese typically talks about the frequency of rainy days rather than the amount of rain when describing a season. Hence 雨の日が多い (“many rainy days”) is the idiomatic choice.

Could I use 春に instead of 春は?

Yes. 春に雨の日が多いです is grammatically correct and means “In spring, there are many rainy days.” However, 春は is more common when you want to generalize about a season as the topic of your statement.