zhème hǎo de jīhuì, nǐ wèishénme bú shìshi?

In this sentence, 这么 means so, such a, or this in the sense of degree:

• 这么好的机会 = such a good opportunity / so good an opportunity

It is showing degree: the opportunity is being described as very good.

A useful pattern is:

• 这么 + adjective + 的 + noun

For example:

• 这么漂亮的地方 = such a beautiful place
• 这么重要的问题 = such an important question

Here, 这么 is not really about physical distance. It is mainly intensifying 好.

的 links the description to the noun that follows.

So:

• 这么好 = so good
• 机会 = opportunity
• 这么好的机会 = such a good opportunity

You can think of 的 as turning the descriptive part into something that modifies the noun.

Structure:

• 这么 + 好 + 的 + 机会

This is very common in Mandarin:

• 很贵的车 = an expensive car
• 我喜欢的书 = the book I like
• 昨天买的衣服 = the clothes bought yesterday

So in your sentence, 的 is simply connecting 这么好 to 机会.

Good question. Both are possible:

• 这么好的机会
• 这么好的一个机会

The version without 一个 is very natural and common. It sounds a bit more streamlined.

Adding 一个 can make it feel a little more specific or concrete:

• 这么好的一个机会 = such a good opportunity / such a great particular opportunity

So:

• 这么好的机会 = natural, common, slightly more general
• 这么好的一个机会 = also natural, slightly more emphatic or specific

Mandarin often omits measure words and numerals when they are not necessary.

This is because of tone sandhi.

Normally, 不 is fourth tone: bù.
But when it comes before another fourth-tone syllable, it changes to second tone: bú.

Here:

• 试 is fourth tone: shì
• so 不试 becomes bú shì

That is why the sentence is written:

• 不{bú} 试试{shìshi}

A few examples:

• 不是 → bú shì
• 不要 → bú yào
• 不去 → bú qù

But before non-fourth tones, it usually stays bù.

试试 is the verb 试 repeated once. This kind of repetition is very common in Mandarin.

• 试 = to try
• 试试 = try it / give it a try / try a bit

Verb reduplication often makes the action sound:

• lighter
• more casual
• less forceful
• more tentative or friendly

So 不试试？ sounds softer and more natural than a plain 不试？ in this context.

Similar examples:

• 看看 = have a look
• 想想 = think about it
• 问问 = ask and see
• 听听 = listen a bit

So 试试 does not mean try twice. It means try it a little / give it a try.

Yes. 试试 and 试一试 are both common and natural.

• 试试 = try it
• 试一试 = try it / give it a try

The meaning is very close. In many situations, they are interchangeable.

A small nuance:

• 试一试 can sound a little more complete or slightly more deliberate
• 试试 can sound a little more conversational and relaxed

So these are both fine:

• 你为什么不试试？
• 你为什么不试一试？

Both are natural Mandarin.

In context, it often works like an English suggestion:

• Why don’t you try?

Even though it literally contains 为什么 (why), the whole pattern 为什么不... is often used to encourage or suggest something.

So it can mean:

• Why don’t you try?
• Why not give it a try?

It does not always sound like the speaker is demanding an explanation. Very often it is just persuasive or encouraging.

A related useful pattern is:

• 为什么不 + verb? = Why not ...?

Examples:

• 为什么不去看看？ = Why not go take a look?
• 为什么不问问他？ = Why not ask him?

So here the sentence is functioning more like a suggestion than a literal request for reasons.

Mandarin often puts the topic first.

Here:

• 这么好的机会 = as for such a good opportunity...
• 你为什么不试试？ = why don’t you try?

So the sentence first sets up the situation, then comments on it.

You can think of it like:

• With such a good opportunity, why don’t you give it a try?

This topic-comment style is very common in Chinese. It helps present the background before the main point.

Yes, that is also grammatical, but the focus is a little different.

Compare:

• 这么好的机会，你为什么不试试？
• 这个机会这么好，你为什么不试试？

The first one emphasizes the noun phrase as a whole:

• such a good opportunity

The second one starts with:

• this opportunity is so good

So the first sounds a bit more compact and idiomatic for this kind of persuasive statement. The second is also fine, but it feels slightly more explicit.

The comma marks a pause between the topic/background and the main comment:

• 这么好的机会， / 你为什么不试试？

It helps show the natural rhythm of the sentence.

In speech, you would usually pause a little after 机会.
In writing, the comma makes the structure clearer.

This kind of comma is very common in Chinese when a sentence begins with a topic, time phrase, or condition.

Yes, but the tone changes.

• 试试 sounds casual, spoken, and encouraging
• 尝试 is more formal and often used in written or careful speech

Compare:

• 你为什么不试试？ = very natural, conversational
• 你为什么不尝试？ = grammatical, but more formal and less idiomatic in everyday speech

In this sentence, 试试 is the more natural choice for friendly spoken Mandarin.

Yes. It is a very useful pattern.

Pattern:

• 这么 + adjective + 的 + noun

Examples:

• 这么好的老师 = such a good teacher
• 这么大的房子 = such a big house
• 这么难的问题 = such a difficult question
• 这么重要的事情 = such an important matter

You can use it whenever you want to say such a ... or so ... a ... in a natural Mandarin way.

It sounds mostly neutral to conversational.

• 这么好的机会 is natural in both speech and writing
• 你为什么不试试？ sounds spoken and friendly because of 试试

So overall, the sentence feels like something someone would naturally say in conversation to encourage another person.

If you wanted to make it more formal, you might choose different wording, but for everyday Mandarin this sentence is very natural.