Learn Hardcore Japanese: nihongo de kaiwa wo suruno ha muzukasii desu. - Having a conversation in Japanese is difficult.

で marks the medium or context in which an action takes place. Here it means “in Japanese” (the language used). Using に would suggest direction or target (“to Japanese”), which doesn’t fit “speaking in Japanese.”

What does 会話をする mean, and why not just 会話?

会話 is a noun meaning “conversation.” To express the action “to have a conversation,” you attach する (“to do”) to the noun and mark it with を. So 会話をする literally means “to do a conversation,” i.e. “to converse.”

Why is there a の after する?

The の here nominalizes the preceding verb phrase (会話をする) into a noun phrase. 会話をするの means “the act of having a conversation.” This allows the phrase to be treated as the topic with は and then described by 難しい.

Can I use こと instead of の for nominalization?

Yes. こと is a more formal or written nominalizer. You can say: 日本語で会話をすることは難しいです。
Both の and こと turn the verb phrase into a noun, with こと sounding slightly more formal.

What role does は play after the nominalized phrase?

After nominalization (会話をするの), は marks that entire phrase as the topic of the sentence. It signals “as for having a conversation in Japanese,” what follows is a comment—namely that it is difficult.

Why is です added after 難しい?

です is the polite copula, making 難しいです a polite statement. In casual speech you can drop です and simply say 難しい.

Could I say 日本語で話すのは難しいです instead of 会話をする?

Yes. 話すの (“the act of speaking”) is a common alternative.
日本語で話すのは難しいです。
This focuses on speaking in general, while 会話をする emphasizes the back‐and‐forth nature of a conversation.

Why is there no subject (like “I” or “we”) in this sentence?

Japanese often omits the subject when it’s clear from context. Here, “it is difficult to have a conversation in Japanese” implies the speaker or people in general without explicitly stating 私 or 私たち.