Я обыскал всю комнату.

Because обыскать and искать are not the same verb.

• искать = to look for something
  
  • Я искал ключи. = I was looking for the keys.
• обыскать = to search a place/person thoroughly
  
  • Я обыскал комнату. = I searched the room.

So in Я обыскал всю комнату, the idea is not just I looked, but I searched the whole room thoroughly.

Also, обыскал is perfective, so it presents the action as completed.

That is the normal marker of the past tense in Russian.

The basic past-tense pattern is:

• masculine: -л
• feminine: -ла
• neuter: -ло
• plural: -ли

So:

• Я обыскал всю комнату. = said by a male speaker
• Я обыскала всю комнату. = said by a female speaker

Russian past tense agrees in gender and number, not person.

Because всю has to agree with комнату, which is in the accusative singular feminine.

The dictionary form is:

• весь = all, whole

Its feminine forms include:

• nominative: вся
• accusative: всю

Since комнату is the direct object here, it is in the accusative, so the adjective must match:

• всю комнату = the whole room

Because комната is the direct object of the verb, so it takes the accusative case.

• nominative: комната = room
• accusative: комнату

This is very common with feminine nouns ending in -а:

• книга → книгу
• машина → машину
• комната → комнату

So:

• Я обыскал комнату = I searched the room

Because with обыскать, the place being searched is a direct object, so no preposition is needed.

Compare:

• обыскать комнату = to search the room
• искать в комнате = to look in the room

These are different structures:

• обыскать + accusative object
• искать + object, or sometimes искать что-то в чём-то

So Russian says directly:

• Я обыскал всю комнату

not something like в комнате in this sentence.

It means the whole room / the entire room.

It refers to one room, searched completely.

It does not mean every room. For that, Russian would use something like:

• все комнаты = all the rooms
• каждую комнату = every room

So:

• Я обыскал всю комнату = I searched the entire room
• Я обыскал все комнаты = I searched all the rooms

Russian word order is more flexible than English word order.

Я обыскал всю комнату is a neutral, natural order:

• Я = I
• обыскал = searched
• всю комнату = the whole room

But other orders are possible, depending on emphasis:

• Всю комнату я обыскал. = emphasizes the whole room
• Я всю комнату обыскал. = also gives extra focus to всю комнату
• Обыскал я всю комнату. = more marked, often stylistic or contrastive

So the basic meaning stays similar, but the focus can shift.

This is a question of aspect.

• обыскал = perfective
• обыскивал = imperfective

Я обыскал всю комнату

This presents the action as completed:

• I searched the whole room
• the search is treated as a finished result

Я обыскивал всю комнату

This focuses on the process:

• I was searching the whole room
• maybe the action was ongoing
• maybe the result is not important

This is a very important Russian contrast.

Yes, it often can be omitted if the context makes it clear.

Russian often drops pronouns when they are understood:

• Обыскал всю комнату.
• Я обыскал всю комнату.

Both can work.

However, in the past tense, the verb does not show person, only gender and number. So without Я, the sentence can sound slightly more dependent on context than in the present tense.

Still, in conversation, omission is very common.

It can be both, but it often sounds more thorough, and sometimes a bit more formal or intensive than a simple look around.

For example, it can be used for:

• searching a room carefully
• searching someone
• police searching a place

So Я обыскал всю комнату suggests a serious, thorough search, not just a casual glance around.

If you wanted something lighter, you might choose a different expression depending on context.

A helpful approximate pronunciation is:

Ya abysKAL fsyu KOMnatu

A few notes:

• Я sounds like ya
• обыскал has the stress on the last syllable: обыска́л
• всю sounds roughly like fsyu
• комнату has stress on the first syllable: ко́мнату

Stress matters in Russian, so it is useful to learn:

• обыска́л
• всю
• ко́мнату