Ngoài bề mặt ra, tớ còn kiểm tra độ dày.

In this pattern, ngoài ... ra means besides, apart from, or in addition to.

So Ngoài bề mặt ra, ... means something like:

• Besides the surface, ...
• Apart from the surface, ...

This is a very common Vietnamese structure:

• Ngoài A ra, B còn ...
• Besides A, B also ...

In your sentence, it introduces one thing that is being considered first (bề mặt), and then adds another thing that is also checked (độ dày).

In this sentence, ngoài ... ra works together as a fixed expression.

• ngoài = outside / besides
• ra helps complete the expression ngoài ... ra, which often means apart from X or besides X

So ngoài bề mặt ra is not just a literal location phrase. It is a grammatical phrase meaning as for the surface, besides that...

If you say only ngoài bề mặt, it will more easily sound like a literal spatial meaning such as outside the surface or on the outside surface, depending on context.

So here, ra is important because it signals the "besides/apart from" structure.

Not in this sentence.

Although ngoài often means outside, here ngoài bề mặt ra is idiomatic and means besides the surface or apart from the surface.

That is why context matters. Compare:

• Ngoài bề mặt ra, tớ còn kiểm tra độ dày.
  = Besides the surface, I also check the thickness.

Versus a more literal spatial use:

• Vết xước nằm ở ngoài bề mặt.
  = The scratch is on the outside surface.

So in your sentence, this is not about physical location. It is about adding another item to consider.

Tớ is an informal first-person pronoun meaning I.

It is commonly used:

• among friends
• with people of similar age
• in casual conversation

It sounds friendly and natural, but not formal.

Other possibilities include:

• mình = very common, softer, flexible
• tôi = neutral/formal
• tao = very casual and can sound rude unless used in the right relationship

So in this sentence, tớ tells you the tone is casual and personal.

Here còn means also, in addition, or furthermore.

In the pattern Ngoài X ra, tớ còn kiểm tra Y, it helps express:

• Besides X, I also check Y

So còn is what makes the second part feel like an added item.

Compare:

• Tớ kiểm tra độ dày.
  = I check the thickness.

• Ngoài bề mặt ra, tớ còn kiểm tra độ dày.
  = Besides the surface, I also check the thickness.

Without còn, the sentence is still understandable in some contexts, but còn makes the additive meaning much clearer and more natural.

Kiểm tra is a very common verb meaning to check, inspect, examine, or sometimes test, depending on context.

It is broader than just one English word. In this sentence, check or inspect is the best fit.

Examples:

• kiểm tra bài tập = check homework
• kiểm tra máy móc = inspect machinery
• kiểm tra chất lượng = check quality

So tớ còn kiểm tra độ dày means the speaker also examines or measures the thickness as part of a checking process.

Because dày is an adjective meaning thick, while độ dày is a noun meaning thickness.

This is an important pattern in Vietnamese:

• dày = thick
• độ dày = thickness

Here, kiểm tra needs a thing to be checked, so a noun works naturally:

• kiểm tra độ dày = check the thickness

You will see the same pattern with other measurable qualities:

• cao = tall / high → độ cao = height
• rộng = wide → độ rộng = width
• sâu = deep → độ sâu = depth

So độ often helps turn an adjective into a noun referring to a measurable degree or dimension.

Yes. Vietnamese often puts a topic or framing phrase at the beginning of the sentence.

So:

• Ngoài bề mặt ra, tớ còn kiểm tra độ dày.

starts by setting up the frame: besides the surface...

Then the main clause follows:

• tớ còn kiểm tra độ dày
• I also check the thickness

This fronted structure is very natural in Vietnamese. It is similar to English sentences like:

• As for the surface, I also check the thickness
• Besides the surface, I also check the thickness

So the word order is not unusual; it is a common way to organize information.

Sometimes in very casual speech, Vietnamese speakers may shorten expressions, but in this structure, ra is normally kept.

So the more standard and natural form is:

• Ngoài bề mặt ra, ...

rather than:

• Ngoài bề mặt, ...

Keeping ra makes the besides/apart from meaning clearer and more idiomatic.

For learners, it is best to learn the full pattern:

• Ngoài X ra, ... còn ...

No. In this sentence, bề mặt is part of the introductory phrase Ngoài bề mặt ra.

The direct object of kiểm tra is:

• độ dày

So the structure is:

• Ngoài bề mặt ra = besides the surface
• tớ còn kiểm tra độ dày = I also check the thickness

If you wanted surface to be the object of kiểm tra, you could say something like:

• Tớ kiểm tra bề mặt.
  = I check the surface.

But that is not the grammatical role it has in your original sentence.

Yes, that is the natural implication.

The phrase Ngoài bề mặt ra suggests that the surface is already one of the things being checked, and the speaker is adding another item:

• the surface
• and also the thickness

That is why the whole sentence feels like:

• I don’t just check the surface; I also check the thickness.

So even though the verb kiểm tra is only stated once in the second clause, the meaning strongly implies both are part of the checking process.

Yes, but it would change the structure slightly.

Ngoài việc is used before a verb phrase, not just a noun.

For example:

• Ngoài việc kiểm tra bề mặt, tớ còn kiểm tra độ dày.

This means:

• Besides checking the surface, I also check the thickness.

This version may actually sound a bit more explicit, because it clearly says checking the surface as an action.

So:

• Ngoài bề mặt ra... = besides the surface
• Ngoài việc kiểm tra bề mặt... = besides checking the surface

Both are natural, but the second one spells out the action more clearly.