Learn Hardcore Chinese: wǒ juéde hēisè de kùzi bǐ báisè de kùzi gèng héshì. - I think black pants are more suitable than white pants.

合适 is usually an adjective:
- 这条裤子很合适。 = “These pants are very suitable / fit well.”
- Pattern: something + 很/更/不 + 合适
适合 is usually a verb (or verb-object):
- 这条裤子适合你。 = “These pants suit you.”
- Pattern: something + 适合 + someone / some purpose

In your sentence:

更合适 = “more suitable” (adjective use), so 合适 is the right choice.
If you want to use 适合, you’d usually say something like:
黑色的裤子比白色的更适合我。
“Black pants suit me more than white pants.”

Can I leave out 我觉得 (wǒ juéde)? Is it necessary to say “I think” in Chinese here?

You can absolutely leave it out:

黑色的裤子比白色的裤子更合适。

This sentence already sounds like an opinion in most contexts, especially when you are obviously giving advice.

Adding 我觉得:

Makes it explicitly your personal view.
Softens the statement (it feels less absolute / less like a fact).

You can also move 我觉得:

我觉得，黑色的裤子比白色的裤子更合适。 (comma pause)
黑色的裤子我觉得比白色的更合适。 (more conversational)

But the version in your sentence, 我觉得 + [whole sentence], is the most standard.

Why use 觉得 (juéde) instead of 想 (xiǎng) or 认为 (rènwéi)?

They all translate as “think” in English, but have different flavors:

觉得:
- Most common, neutral, everyday “I feel / I think (in my opinion).”
- Natural for opinions, preferences, impressions.
- 我觉得这个颜色不错。
想:
- Often “want to / intend to / think about.”
- As “think,” it’s more like “I think (that might be the case)” but not used as generally for opinions as 觉得.
- In this sentence, 我想黑色的裤子比白色的裤子更合适 is possible, but 我觉得 is more usual.
认为:
- More formal, like “I consider / I hold the view that.”
- Used in written language, debates, reports.
- 我认为这个方案更合适。

For choosing clothes and stating a preference, 我觉得 is the most natural choice.

Do I need a measure word like 条 (tiáo) for 裤子 (kùzi) in this sentence?

You don’t need a measure word here because you are talking about pants as a category, not a specific number of items.

Generic / category:
- 黑色的裤子比白色的裤子更合适。
  “Black pants (as a type) are more suitable than white pants.”

If you want to talk about one pair specifically, you would use 条:

我觉得这条黑色的裤子比那条白色的裤子更合适。
“I think this pair of black pants is more suitable than that pair of white pants.”

Can I say 黑色的裤子更合适比白色的裤子 or put 比 at the end?

No. The word order must follow:

A + 比 + B + (更) + adjective

So:

✅ 黑色的裤子比白色的裤子更合适。
❌ 黑色的裤子更合适比白色的裤子。
❌ 黑色的裤子更合适，白色的裤子比。 (ungrammatical)

Think of 比 as a preposition that introduces what you are comparing against. It always comes between A and B, never at the end.

How would I say the opposite, like “White pants are not as suitable as black pants”?

You can express that using a negative comparison:

A 没有 B + (这么/那么) + adjective
- 白色的裤子没有黑色的裤子那么合适。
  “White pants are not as suitable as black pants.”
Or flip A and B with 比 and add 不:
- 白色的裤子比黑色的裤子不合适。
  Grammatically possible, but the first pattern with 没有 is more natural in this “not as … as …” meaning.

Your original sentence is the positive version:

黑色的裤子比白色的裤子更合适。

What exactly does 裤子 (kùzi) mean? Is it singular or plural?

裤子 means “trousers / pants” as an item of clothing. Chinese doesn’t mark singular vs. plural the way English does:

一条裤子 = one pair of pants
两条裤子 = two pairs of pants
裤子 on its own can mean “pants” in general or “a pair of pants,” depending on context.

In your sentence, 黑色的裤子 and 白色的裤子 refer to pants as a general type, not a specific counted number.

How is 觉得 (juéde) pronounced? Why is the second syllable neutral?

觉得 is pronounced juéde:

觉 (jué) = second tone
得 (de) here is in neutral tone (light, unstressed), not the usual second or third tone of 得 in other words.

Neutral tone syllables are very common in high-frequency words and grammatical particles. In 觉得, the de is unstressed and short, so we mark it as neutral: juéde, not juédé or juédé.