Learn Hardcore Japanese: katyou no setumei ha wakariyasui desu. - The section chief’s explanation is easy to understand.

In Japanese, の links two nouns in a possessive or descriptive way. Here, 課長の説明 literally means “the explanation of the section manager.” Without の, you can’t smoothly connect those two nouns.

What function does は serve after 説明?

は marks the topic of the sentence. By saying 課長の説明は…, you’re indicating you’re talking about that explanation specifically, and then you give a comment about it (“is easy to understand”).

Could we use が instead of は and say 課長の説明が分かりやすいです? What’s the difference?

Yes, you can. Using が tends to introduce new or focused information: “It is the manager’s explanation that is easy to understand.” Using は makes a more general statement about a known topic: “As for the manager’s explanation, it’s easy to understand.” The nuance with が is stronger focus on the subject, while は feels more like describing something you’re already discussing.

What kind of adjective is 分かりやすい, and how does it interact with です?

分かりやすい is an い-adjective (i-adjective). In casual speech you can just say 分かりやすい, but in polite speech you add です: 分かりやすいです. The adjective itself doesn’t lose its い; you simply tack です onto the end for politeness.

Why is です necessary here? Can you omit it?

です adds politeness and completes the sentence in a formal register. In casual contexts (with friends, for example), you can omit it and just say 分かりやすい. But in most business or polite situations you keep です.

Why is 分かりやすい partly in kanji and partly in hiragana?

The verb root 分かり comes from 分かる and is commonly written with the kanji 分 plus hiragana かり. The suffix 〜やすい is always in hiragana when forming an i-adjective. So the standard spelling is 分かりやすい.

Why can’t I say 課長説明は分かりやすいです without の?

Japanese generally requires a particle—often の—to link two nouns. Omitting の between 課長 and 説明 sounds unnatural and unclear. 課長の説明 clearly shows that the explanation belongs to or is given by the section manager.

Is it possible to change the word order, for example, 分かりやすいです、課長の説明?

Standard Japanese follows a topic-comment (SOV) order: 課長の説明は分かりやすいです. While in very casual speech you might hear unusual orders for effect, it’s best to stick to the normal pattern in writing and polite conversation.