Learn Hardcore Chinese: tā juéde zhè gè shìqíng zuì zhòngyào. - She thinks this matter is the most important.

The most idiomatic choice with 事/事情 is 件: 这件事 or 这件事情.
这个事情 is widely heard in Mainland colloquial speech and is acceptable, but many teachers prefer 这件事 as the standard.
Short and very common: 这个事 or 这件事. If you want the safest, most neutral phrasing, use 这件事.

Can I say 这事情 without a measure word?

You’ll hear 这事情 in some regions, but in standard Mandarin, a demonstrative like 这/那 normally takes a measure word: say 这件事 (best) or 这个事 (colloquial).

What’s the difference between 事, 事情, and 事儿?

事 (shì): general “matter/affair”; very common in both speech and writing. Example: 这件事.
事情 (shìqíng/shì·qing): fuller form; sounds a bit more formal or explicit; also very common.
事儿 (shìr): colloquial with erhua (northern flavor). Common in Beijing/colloquial speech: 这事儿 or 这件事儿.

Do I need 是 before 最重要? Should I say 她觉得这件事是最重要的?

她觉得这件事最重要: perfectly natural; adjective as predicate, no 是 needed.
她觉得这件事是最重要的: also correct; adds emphasis or a clearer “the most important one” sense. The 的 nominalizes the adjective.
Avoid 是最重要 without 的; it sounds incomplete.

Why is there no 的 after 重要 here?

Predicative adjectives don’t take 的: 这件事最重要.
Use 的 when the adjective modifies a noun or is nominalized:
- Attributive: 最重要的事情 = “the most important matter”
- Nominalized: 最重要的是… = “The most important thing is …”

Should there be 很 before 重要? I learned adjectives need 很.

With a bare adjective, Mandarin often inserts a degree adverb like 很: 这件事很重要.
But with a superlative 最, you do not add 很: 最重要 already marks degree. So your sentence is fine without 很.

What’s the difference between 觉得, 认为, 以为, and 感觉?

觉得 (juéde): “feel/think” based on subjective impression; very common and neutral.
Example: 我觉得这件事很重要。
认为 (rènwéi): “hold the opinion that”; more reasoned/formal.
Example: 专家认为这个计划可行。
以为 (yǐwéi): “assume/think (but be wrong)”. Often implies a mistaken belief.
Example: 我以为他会来，结果没来。
感觉 (gǎnjué): as a verb “to feel/sense” (physical/emotional), or a noun “feeling”. As an opinion verb it’s less direct than 觉得.
Example: 我感觉他最近压力很大。

Any pronunciation pitfalls in this sentence?

觉得 is juéde; the second syllable is neutral tone (not júe‑dé). Character: 得, pronounced neutral here.
这个 is zhège; in colloquial speech you’ll also hear zhèi ge; 个 is often a neutral tone here.
事情 may be pronounced shìqíng (standard) or with a neutral second syllable shì·qing in casual speech.
最重要 is zuì zhòngyào (4th + 4th + 4th); there’s no mandatory tone sandhi change.
The 重 in 重要 is zhòng (not chóng).

Does 最 need an “of/in …” phrase like English “the most … of all”?

No. 最 can stand alone when the comparison set is obvious from context. If you want to spell it out, add a phrase:

在所有选项里，这件事最重要。 = “Among all the options, this is the most important.”
对她来说，这件事最重要。 = “For her, this is the most important.”

How do I say “least important” or “not the most important”?

Least important: 最不重要.
Example: 她觉得这个细节最不重要。
Not the most important: 不是最重要的.
Example: 她觉得这件事不是最重要的。
To negate the opinion itself: 她不觉得这件事最重要。

How do I express time (past/present) with 觉得 since there’s no tense?

Add time expressions or aspect where appropriate:

Past viewpoint: 她以前/当时/那时候觉得这件事最重要。
Present viewpoint: 她现在觉得这件事最重要。
Change of stance: 她觉得这件事最重要了 (now she thinks it’s most important; indicates a change).

Can I change the word order, like topicalizing the thing she thinks is important?

Yes. Topic–comment structures are natural:

这件事，她觉得最重要。
Or focus perspective: 对她来说，这件事最重要。
The meaning stays the same; the emphasis shifts to the topic or the viewpoint.

Can I drop 她 or 这 if context is clear?

Yes, subject and topic dropping is common in discourse:

If “she” is already known: 觉得这件事最重要。
If the matter is known: 她觉得最重要。
Be careful in writing to maintain clarity.

Any quick alternatives to 重要, and their feel?

关键 (guānjiàn): “crucial/key”; slightly stronger or more specific than 重要.
Example: 她觉得这件事最关键。
要紧 (yàojǐn): colloquial “urgent/important.”
Example: 这件事很要紧。
Very formal: 最为重要.
Example: 她认为这件事最为重要。

How would I say “She thinks this is more important (than something else)”?

Use 更 and optionally make the comparison explicit:

Implicit: 她觉得这件事更重要。
Explicit: 她觉得这件事比别的事更重要。