Он слегка открыл дверь.

Открыть is the infinitive, meaning to open.
In the sentence, we need a finite verb, not the dictionary form, because the action actually happened.

• открыть = to open
• открыл = opened (past tense, masculine singular)

So Он слегка открыл дверь means He slightly opened the door / He opened the door a little.

In Russian, the past tense is usually formed with -л.

For the verb открыть, the past tense forms are:

• он открыл = he opened
• она открыла = she opened
• оно открыло = it opened
• они открыли = they opened

So the -л is a normal marker of the past tense, and here the form is masculine because it goes with он.

Он means he.

Russian often allows the subject pronoun to be omitted if it is clear from context, so in some situations you could also just say:

• Слегка открыл дверь.

But using он makes the subject explicit. It can help with clarity, contrast, or emphasis.

Yes, дверь is the direct object, so it is in the accusative case.

The reason it looks the same as the nominative is that дверь is an inanimate feminine noun ending in -ь, and for many such nouns the nominative singular and accusative singular are identical:

• nominative: дверь = door
• accusative: дверь = door

So the case is accusative by function, even though the form does not change.

Слегка means slightly, a little, or lightly, depending on context.

In this sentence, it means that he did not open the door fully. He opened it only a bit.

So the idea is something like:

• He opened the door slightly
• He opened the door a little
• He opened the door just a bit

Both can often be translated as a little, but they are not always identical.

• слегка often suggests a slight degree or lightly
• немного more generally means a little or not much

In this sentence:

• Он слегка открыл дверь = He opened the door slightly
• Он немного открыл дверь = He opened the door a little

Both are possible, but слегка can sound a bit more like just slightly.

The verb открыть is perfective. That means it presents the action as a whole, as a completed event.

Here that makes sense because the sentence describes one finished action: he opened the door a little.

The imperfective partner is открывать.

Compare:

• Он слегка открыл дверь. = He opened the door slightly.
  A single completed action.

• Он слегка открывал дверь. = He was opening / used to open the door slightly.
  This sounds unusual without more context, because imperfective focuses more on process, repetition, or background action.

So открыл is the natural choice here.

Usually it means he opened it only a little — not all the way.

That is the most natural interpretation of слегка открыл дверь.

Depending on context, слегка can sometimes suggest a gentle or slight manner, but here the main idea is the degree of opening, not the force.

So think:

• door opened a crack
• door opened slightly
• door opened partway

Yes. A very common alternative is:

• Он приоткрыл дверь.

This often means He opened the door slightly / He opened the door a crack.

Compared with слегка открыл дверь, приоткрыл builds the idea of opening just a little directly into the verb itself.

So:

• слегка открыл дверь = literally slightly opened the door
• приоткрыл дверь = opened the door a little / ajar

Both are good, but приоткрыл is especially natural for opened a bit.

Yes. Russian word order is flexible, although the neutral order here is very natural:

• Он слегка открыл дверь.

Other orders are possible, but they can change emphasis:

• Слегка он открыл дверь.
  Emphasizes slightly more strongly; less neutral.

• Дверь он слегка открыл.
  Emphasizes the door.

• Он дверь слегка открыл.
  Also possible, with a more conversational feel.

So the original sentence is a straightforward, neutral way to say it.

The stress is:

• Он слегкá откр ы́л дверь.

More clearly:

• слегкá
• откр ы́л
• дверь (one syllable)

A rough pronunciation guide is:

• on slyig-KA at-KRYL dver'

The final ь in дверь is a soft sign, so the final consonant is softened.

Yes, дверь is a feminine noun.

Its gender matters for agreement in other parts of the sentence when needed. For example:

• тяжёлая дверь = a heavy door
• дверь открыта = the door is open

But in Он слегка открыл дверь, the verb открыл agrees with он, not with дверь, because он is the subject and дверь is the object.

So the masculinity of открыл comes from он, not from дверь.