Он посмотрел внутрь коробки.

Because посмотрел is perfective, while смотрел is imperfective.

• посмотрел = he looked / he took a look
• смотрел = he was looking / he looked for some time

In this sentence, the action is seen as one complete act: he looked inside the box. That is why посмотрел is natural.

Compare:

• Он посмотрел внутрь коробки. = He looked inside the box.
• Он смотрел внутрь коробки несколько минут. = He was looking inside the box for several minutes.

внутрь means inside, into the inside, or more literally inward.

In this sentence, it shows the direction of the gaze: he looked into the inside of the box.

A useful contrast is:

• внутрь = to the inside / inward
• внутри = inside / on the inside as a location

So:

• Он посмотрел внутрь коробки. = He looked inside the box.
• Что внутри коробки? = What is inside the box?

Because внутрь is used here with the genitive case, and the genitive singular of коробка is коробки.

So:

• dictionary form: коробка = box
• genitive singular: коробки

That is why you get:

• внутрь коробки

This is a common pattern with words expressing movement or direction toward an interior area.

Yes, absolutely. Он посмотрел в коробку is very natural and common.

Both sentences can mean He looked inside the box:

• Он посмотрел внутрь коробки.
• Он посмотрел в коробку.

The difference is mostly one of style and emphasis:

• в коробку is simpler and more everyday
• внутрь коробки puts a little more focus on the interior

So a learner should definitely understand both, but in everyday speech в коробку may be more common.

It can behave like both, depending on the sentence.

In Он посмотрел внутрь коробки, it functions in a preposition-like way because it is followed by a noun in the genitive: коробки.

But внутрь can also appear on its own, more like an adverb:

• Он заглянул внутрь. = He looked inside.

So it is best to think of внутрь as a word meaning inward/inside, which can either stand alone or combine with a noun.

Because Russian does not have articles like a and the.

So коробка, коробки, в коробку, etc. can mean:

• a box
• the box

The exact meaning depends on the context.

That means Он посмотрел внутрь коробки could mean:

• He looked inside the box.
• He looked inside a box.

Usually the surrounding context tells you which one is meant.

Он is in the nominative case because it is the subject of the sentence.

• он = he

So the structure is:

• Он = subject
• посмотрел = verb
• внутрь коробки = where he looked

This is the normal basic setup of a Russian sentence.

This word order is neutral and natural:

• Он посмотрел внутрь коробки.

Russian word order is more flexible than English word order, because case endings show grammatical relationships. But different word orders can shift the emphasis.

For example:

• Он посмотрел внутрь коробки. = neutral
• Внутрь коробки он посмотрел. = emphasizes inside the box
• Посмотрел он внутрь коробки. = can sound more narrative or stylistically marked

So the original sentence is a good standard word order for learners.

A very useful shortcut is:

• внутрь = into / inward → direction
• внутри = inside → location

Examples:

• Он посмотрел внутрь коробки. = He looked into the box.
• Игрушка была внутри коробки. = The toy was inside the box.

Even though English often uses inside for both ideas, Russian keeps the distinction more clearly.

The stress is:

• Он посмотре́л внутрь коро́бки.

A rough pronunciation guide:

• Он = on
• посмотре́л = puh-smuh-TRYEL
• внутрь = VNOO-tr’
• коро́бки = kuh-ROB-kee

A couple of important points:

• посмотре́л has stress on the last syllable
• внутрь is a short word with a final soft consonant
• коро́бки keeps the stress on ро́

If you want to sound natural, make sure the stressed syllables are clear: -тре́л, вну-, -ро́б-.