Вряд ли оса залетит в комнату, если окно закрыто.

Вряд ли is a fixed expression meaning hardly, unlikely, or probably not.

In this sentence, Вряд ли оса залетит... means It’s unlikely that a wasp will fly in...

It is normally written as two words:

• вряд — not really used on its own in modern speech
• ли — a particle

So it is best to learn вряд ли as one set phrase.

Because вряд ли already gives the sentence a negative meaning: unlikely.

So:

• Вряд ли оса залетит = The wasp is unlikely to fly in

If you added не, you would change the meaning:

• Вряд ли оса не залетит = It’s unlikely that the wasp won’t fly in
  which suggests it probably will fly in.

So in the original sentence, no не is needed.

Because оса is the subject of the sentence — it is the thing doing the action.

Dictionary form:

• оса = wasp

Here:

• оса залетит = the wasp will fly in

So nominative is used because it answers who will fly in? — оса.

Залетит is the future tense of the perfective verb залететь.

That is used because the sentence is talking about a single completed event:

• the wasp flies into the room

Compare:

• летит = is flying / flies (imperfective, more ongoing or general)
• залетит = will fly in / will get into (perfective, one completed action)

So залетит fits the idea of entering the room as a complete event.

Here за- gives the idea of flying into / getting into a place.

So:

• лететь = to fly
• залететь = to fly in, fly into, get into by flying

With insects or birds, залететь often suggests entering somewhere, sometimes accidentally:

• Птица залетела в дом = A bird flew into the house
• Муха залетела в окно = A fly flew in through the window

So in this sentence, оса залетит в комнату means a wasp will fly into the room.

Because this sentence expresses movement into a place.

After в:

• в + accusative = motion into
• в + prepositional = location in

So:

• в комнату = into the room
• в комнате = in the room

Since the wasp would be moving into the room, Russian uses the accusative:

• комната → комнату

Because закрыто is the short form used as a predicate: is closed.

Compare:

• закрытое окно = a closed window
  This is a full adjective describing a noun directly.
• окно закрыто = the window is closed
  This is a predicate statement.

Since окно is neuter singular, the short form is:

• закрыт — masculine
• закрыта — feminine
• закрыто — neuter
• закрыты — plural

So:

• окно закрыто = the window is closed

In Russian, after если (if), the present tense is often used for a real or general condition, even when the main clause refers to the future.

So:

• если окно закрыто literally = if the window is closed
• whole sentence = A wasp is unlikely to fly into the room if the window is closed

This is normal Russian. English often does something similar:

• If the window is closed, the wasp won’t get in.

Because если окно закрыто is a subordinate clause.

Russian normally separates subordinate clauses with a comma:

• Вряд ли оса залетит в комнату, если окно закрыто.

This is the standard punctuation rule.

Yes. Russian word order is flexible.

For example, these are all possible:

• Вряд ли оса залетит в комнату, если окно закрыто.
• Оса вряд ли залетит в комнату, если окно закрыто.
• Если окно закрыто, вряд ли оса залетит в комнату.

The basic meaning stays the same, but the emphasis changes slightly.

The original version sounds natural and starts with вряд ли, which immediately signals unlikelihood.

Yes, usually.

• наверное, не = probably not
• вряд ли = unlikely / hardly likely

Вряд ли often sounds a bit firmer or more definite about the low probability.

So:

• Наверное, оса не залетит = The wasp probably won’t fly in
• Вряд ли оса залетит = It’s unlikely the wasp will fly in

The stress is on the last syllable:

• залети́т

A rough pronunciation guide:

• za-lye-TEET

Stress matters in Russian, so it is good to learn the whole phrase as:

• Вряд ли оса залети́т в ко́мнату...

Залететь is very natural here, especially for insects, birds, or anything that flies into a place.

Possible alternatives exist, but they sound different:

• влететь = to fly into, often more direct or forceful
• попасть в комнату = to get into the room, more general
• залететь = very natural for a wasp accidentally flying in

So for a sentence about a wasp coming into a room through a window, залететь is an excellent choice.