zhè zhī qiānbǐ tài duǎn le, bùrú xiān yòng xiàngpí pángbiān nà zhī xīn de ba.

Because 支 is the measure word (classifier) for things like pens, pencils, brushes, and other long, stick-like objects.

So:

• 这支铅笔 = this pencil
• 那支新的 = that new one

In Mandarin, when you use a demonstrative like 这 (this) or 那 (that) with a noun, you usually need a classifier in between:

• 这 + 支 + 铅笔
• 那 + 支 + 铅笔

The second 支 is still needed even though 铅笔 is omitted there.

Because the normal Mandarin order is:

demonstrative + classifier + noun

So:

• 这支铅笔 = this pencil
• 那支笔 = that pen
• 这本书 = this book

English does not use classifiers this way, but Mandarin does, so the order is very fixed.

太 + adjective + 了 is a very common pattern meaning too... or so...

Here:

• 太 = too / excessively
• 短 = short
• 了 = adds emphasis to the situation

So 太短了 means too short.

This is not past tense. The 了 here is not saying the pencil became short in the past. It is just marking the speaker’s judgment or reaction: it’s too short.

Literally, 不如 often means not as good as or not equal to.

For example:

• 这个不如那个好。 = This one is not as good as that one.

But in this sentence, 不如 + verb phrase is being used more like:

• it would be better to...
• why not...
• might as well...

So:

• 不如先用...吧 means something like it would be better to use ... first or why not use ... first.

It sounds like a suggestion based on the situation.

Because 先 is an adverb, and adverbs usually come before the verb in Mandarin.

So:

• 先用 = use first
• 先看 = look first
• 先吃 = eat first

The order is very natural in Chinese:

• 不如先用... = it would be better to first use...

A natural way to understand it is:

• 橡皮旁边 = beside the eraser / next to the eraser
• 那支新的 = that new one

Together:

• 橡皮旁边那支新的 = that new one next to the eraser

Chinese often puts location information before the noun phrase it describes.

So this structure is similar to expressions like:

• 桌子上那本书 = that book on the table
• 门口那个人 = that person at the door

A more expanded version of the same idea would be something like:

• 在橡皮旁边的那支新铅笔

But the original sentence is shorter and very natural in conversation.

Because the noun is being left out.

• 那支新的 literally means that new one
• The omitted noun is 铅笔

So the full idea is:

• 那支新的(铅笔)

In Mandarin, adjective + 的 can stand in for the ... one when the noun is understood.

Compare:

• 那支新铅笔 = that new pencil
• 那支新的 = that new one

So here, 的 helps turn 新 into something like new one.

Yes.

• 那支新铅笔 = that new pencil
• 那支新的 = that new one

The version in the sentence does not repeat 铅笔, because it is already obvious from context. Chinese often leaves out repeated nouns when the meaning is clear.

That makes the sentence sound more natural and less repetitive.

吧 softens the tone and makes the sentence sound like a suggestion.

So:

• 不如先用橡皮旁边那支新的吧。

feels like:

• Why not use the new one next to the eraser first?
• Let’s use the new one next to the eraser first.

Without 吧, the sentence would sound more direct or firm. With 吧, it sounds gentler and more conversational.

Because Mandarin often omits words that are already clear from context.

Once the speaker has said 这支铅笔 (this pencil), it is easy to understand that 那支新的 also refers to a pencil.

So instead of saying:

• 那支新的铅笔

the speaker can simply say:

• 那支新的

This kind of omission is very common in Chinese.

Yes. In this sentence, 橡皮 means eraser.

In modern Mandarin, especially in Mainland China:

• 橡皮 = eraser
• 橡皮擦 = eraser, more explicit/full form

So:

• 橡皮旁边 = next to the eraser

If you are a British English speaker, it may help to remember that this is the ordinary eraser/rubber for a pencil, not something else.