Learn Hardcore Japanese: nihongo no mondai ha muzukasii desu.

The particle の links two nouns, showing that the first noun modifies the second. Here it turns 問題 into “a problem/question about Japanese.” In English it’s like saying “Japanese-language problem.”

Why does 問題 have は after it instead of が?

は marks the topic of the sentence—the thing we’re talking about. By saying 日本語の問題は…, you’re framing “Japanese questions” as the topic and then commenting that they’re difficult. が would simply mark the grammatical subject without introducing a broader topic or contrast.

Can I replace は with が and say 日本語の問題が難しいです?

Yes, you can. Using が puts slightly more focus on 問題 as the specific subject and is often used to answer questions like 何が難しいですか？ (“What is difficult?”). With が, it feels like “It is the Japanese questions that are difficult,” whereas は carries a more general or contrasting tone.

Why is 難しい followed by です?

難しい is an い-adjective meaning “difficult.” Adding です makes your statement polite. Without です, 難しい is perfectly grammatical but casual/informal: 問題は難しい.

Is 日本語の問題は難しいです formal or informal? How would I say it casually?

It’s polite (formal) because of です. To make it casual, you simply drop です:
日本語の問題は難しい。

What kind of adjective is 難しい, and how do い-adjectives work at the end of sentences?

難しい is an い-adjective, which can directly predicate a sentence (no extra verb needed). In casual speech you say 〜難しい, in polite speech you add です to the adjective: 難しいです.

Why are there spaces between each word in 日本語　の　問題　は　難しい　です? Do Japanese texts usually have them?

Native Japanese writing doesn’t use spaces between words. The spaces here are only for learners to see each element clearly. In real books, newspapers, apps, etc., words run together without spaces.

Why is there no word for “the” or “a” as in English—why not the Japanese problem?

Japanese has no articles like “a” or “the.” Definiteness and number come from context. 日本語の問題は難しいです could mean “(The) Japanese problem is difficult,” “Japanese problems are difficult,” or simply “Japanese questions are difficult,” depending on the situation.

Does 問題 imply plural? Should I translate it as “problems” instead of “problem”?

Japanese nouns normally don’t change form for singular or plural. 問題 can be singular or plural. If you know there are multiple questions, you’d translate it as “problems” or “questions.” If it’s just one, you’d say “problem/question.”

Why can’t I omit の and write 日本語問題は難しいです?

That sounds unnatural because 日本語問題 isn’t an established compound in Japanese. To link two independent nouns in a general way, you almost always need の. Without の, native speakers would be confused.