èr hé sān bù yíyàng.

Because this sentence is talking about the number 2 itself, not using 2 to count objects.

A quick rule of thumb:

• 二 is used for the number 2 in counting, math, phone numbers, dates, and when naming the number itself.
• 两 is usually used before a measure word or noun, like 两个 (two items) or 两个人 (two people).

So when comparing the numbers 2 and 3, 二和三 is correct.

Here, 和 means and.

So:

• 二和三 = two and three

It links the two things being compared. In other contexts, 和 can sometimes be translated as with, but here it is clearly the conjunction and.

Mandarin often does not need a word like English is / are in sentences like this.

In 二和三不一样, the phrase 不一样 works as the predicate:

• 不 = not
• 一样 = the same

So the sentence is literally something like:

• Two and three not same

Natural English: Two and three are different.

Mandarin commonly leaves out is/are in this kind of structure.

不一样 means not the same, which is usually translated naturally as different.

Breakdown:

• 不 = not
• 一样 = same / alike

So:

• 一样 = the same
• 不一样 = different

This is a very common everyday way to say that two things are different.

Because 不 normally goes directly before the word or phrase it negates.

Here it negates 一样:

• 一样 = the same
• 不一样 = not the same

This is very normal Mandarin word order. The pattern is:

• A 和 B 不一样 = A and B are different

This is because of a tone change rule for 一.

Normally, 一 is first tone: yī.
But before a fourth-tone syllable, it changes to second tone: yí.

Since 样 is fourth tone (yàng), 一样 is pronounced:

• yíyàng

This is a very common tone sandhi rule.

不 changes from bù to bú only when it comes before another fourth-tone syllable.

For example:

• 不是 = bú shì

But in 不一样, the next pronounced syllable is yí, which is second tone, not fourth tone. So it stays:

• bù yíyàng

Even though 一 is underlyingly yī, what matters here is the actual pronunciation in the phrase.

Yes. That is also natural.

• 和 and 跟 can both mean and / with in this kind of comparison.
• 二和三不一样
• 二跟三不一样

Both mean the same thing here.
For beginners, 和 is often taught first, but 跟 is also very common in spoken Mandarin.

Yes, you could.

• 不一样 = different / not the same
• 不同 = different

So:

• 二和三不一样
• 二和三不同

Both are correct. In many everyday situations, 不一样 sounds a little more conversational and common, while 不同 can sound a bit more formal or concise.

No. It can be used in both positive and negative sentences.

For example:

• 这两个一样。 = These two are the same.
• 这两个不一样。 = These two are different.

It also appears in comparison patterns such as:

• A 跟 B 一样 = A is the same as B

So 一样 is a very useful word beyond this one sentence.

Mandarin often puts the things being talked about first, and then says something about them.

So the structure here is:

• 二和三 = the two things being discussed
• 不一样 = are different

That gives:

• 二和三不一样

This is the normal and natural order for a basic statement like this.

It is grammatically natural, but it feels a bit like a textbook example because the meaning is so obvious: of course 2 and 3 are different.

Teachers use sentences like this because they are simple and clearly show the pattern:

• A 和 B 不一样

So while a native speaker could say it, its main value is as a clean grammar example.