Kun jij dit document even bekijken voordat we het opsturen?

Because this is a yes/no question, and Dutch normally puts the finite verb first in that kind of sentence.

• Statement: Jij kunt dit document even bekijken.
• Question: Kun jij dit document even bekijken?

With jij, the verb kunnen often changes to kun when it comes before the subject. That is why you get Kun jij ...?

You may also hear Kan jij ...? in everyday Dutch, especially in the Netherlands, but Kun jij ...? is a very standard and safe form for learners.

Both mean you in the informal singular.

• je = unstressed, more neutral, very common
• jij = stressed, fuller form, often a bit more emphatic

So:

• Kun je dit document even bekijken? = very natural, neutral
• Kun jij dit document even bekijken? = can you look at it? / slightly more emphasis on the person

In real speech, jij does not always sound strongly emphatic, but it can suggest you specifically a little more than je.

Here even does not literally mean even in the English sense.

In requests, even is a very common softener. It often means something like:

• just
• for a moment
• quickly
• or simply a polite softening word with no exact English equivalent

So dit document even bekijken is like saying:

• just have a quick look at this document
• take a moment to look over this document

It makes the request sound more friendly and less direct.

Because document is a het-word in Dutch.

For singular nouns:

• de-word → deze
• het-word → dit

So:

• deze tafel
• dit document

That is also why the later pronoun is het.

Because het refers back to dit document.

Since document is a het-word, the pronoun used for it is also usually het:

• dit document → het

So voordat we het opsturen means before we send it off.

Bekijken literally means to look at, but in this context it often means something like:

• look over
• review
• check
• have a look at

So the idea is not just physically seeing the document, but examining it briefly before it is sent.

It is broader than just zien (to see) and more natural here than kijken naar, because the speaker is asking someone to inspect or review the document.

Because voordat introduces a subordinate clause.

In Dutch subordinate clauses, the finite verb normally goes toward the end of the clause.

So:

• Main clause: We sturen het op.
• Subordinate clause: voordat we het opsturen

That final verb position is one of the most important Dutch word-order patterns to get used to.

Because opsturen is a separable verb.

Its parts are:

• op
  • sturen

In a main clause, separable verbs split:

• We sturen het op.

In a subordinate clause, they usually stay together:

• ..., voordat we het opsturen.

So the sentence is showing a very common contrast in Dutch:

• Ik stuur het op.
• Ik weet dat ik het opstuur.

It is a present-tense finite verb, not an infinitive.

It only looks like an infinitive because with we the present-tense plural form is the same as the infinitive:

• infinitive: opsturen
• present tense with we: wij/we opsturen

Compare:

• Wij sturen het op.
• ..., voordat wij het opsturen.

And with another subject you can see the difference more clearly:

• ..., voordat hij het opstuurt

There the present-tense form is clearly not the infinitive.

It is informal and friendly.

Why?

• jij = informal you
• even = softens the request
• the whole sentence sounds like a normal workplace or everyday request to someone you know

A more formal version would be:

• Kunt u dit document even bekijken voordat we het opsturen?

That would be suitable when speaking to one person formally.

Yes, very easily. Some common alternatives are:

• Kun je dit document even bekijken voordat we het opsturen?
• Wil je dit document even bekijken voordat we het opsturen?
• Zou je dit document even kunnen bekijken voordat we het opsturen?

These all mean roughly the same thing, but with slightly different tones:

• Kun je ...? = neutral, common
• Wil je ...? = often sounds like would you mind...
• Zou je ... kunnen ...? = a bit softer and more polite

So the original sentence is very natural, but it is only one of several common ways to make this kind of request.