Я недолго ждал у входа в театр.

Here недолго means not for long or for a short time. In natural English, this sentence is often translated as I didn’t wait long at the entrance to the theater.

It describes the duration of the waiting, not the result of the action.

In this sentence, недолго is normally written as one word because it functions as a single adverb meaning for a short time.

A useful rule of thumb:

• недолго = a regular adverb with the meaning for a short time
• не долго = separate spelling when the speaker is emphasizing the negation itself or making a contrast

For example:

• Я недолго ждал. = I waited only a short time.
• Я ждал не долго, а всего пару минут. = I didn’t wait long, only a couple of minutes.

In your sentence, the fused form недолго is the most natural.

Ждал is the past tense masculine singular form of ждать.

Russian past tense agrees with the subject in gender and number:

• я ждал = I waited if the speaker is male
• я ждала = I waited if the speaker is female
• мы ждали = we waited

So if a woman were speaking, the sentence would be:

Я недолго ждала у входа в театр.

Russian uses aspect to show how the speaker views the action.

• ждал is imperfective
• подождал is perfective

In this sentence, ждал is natural because the speaker is talking about the process or duration of waiting: it lasted not long.

Подождал is also possible in some contexts, but it has a slightly different feel: it presents the waiting as a completed whole action, often waited for a bit.

Compare:

• Я недолго ждал... = focus on the fact that the waiting did not last long
• Я немного подождал... = focus on a short completed act of waiting

So ждал works very well with недолго.

Yes, Russian could omit я if the context already makes the subject clear:

• Недолго ждал у входа в театр.

Russian often drops subject pronouns because the verb form already gives information. But in the past tense, ждал only shows masculine singular, not specifically I, so я can help make the subject clearer.

Including я is completely normal. It can also make the sentence feel a bit more explicit or contrastive.

After the preposition у, Russian uses the genitive case.

So:

• вход = nominative
• у входа = genitive after у

Here у входа means by the entrance, near the entrance, or at the entrance.

This is a very common pattern:

• у дома = by the house
• у двери = by the door
• у станции = by the station

So у входа is not random; it is exactly what Russian grammar requires after у.

This is a very common question, because в театр looks like motion toward a place, even though the whole sentence is not about going anywhere.

The key is that в театр depends on the noun вход:

• вход в театр = entrance to the theater

So the phrase means literally at the entrance into the theater.

After вход, Russian commonly uses в + accusative to show where the entrance leads:

• вход в дом = entrance to the house
• вход в парк = entrance to the park
• вход в театр = entrance to the theater

Here театр is accusative singular, but because it is an inanimate masculine noun, its accusative form is the same as the nominative:

• nominative: театр
• accusative: театр

So в театр is correct.

It usually means at/by the entrance in a practical sense.

The preposition у often suggests near, by, or right next to, rather than literally being inside something. So у входа в театр gives the image of someone waiting by the theater entrance.

In many contexts, English simply says at the entrance, but Russian у is slightly more spatially concrete.

Yes. Russian word order is fairly flexible, and changing it usually changes the focus or emphasis, not the basic meaning.

Your sentence is neutral and natural:

• Я недолго ждал у входа в театр.

Other possible orders:

• У входа в театр я недолго ждал.
  Focuses more on where I was waiting.

• Я у входа в театр недолго ждал.
  Also possible, though a bit less neutral.

• Недолго я ждал у входа в театр.
  Stronger emphasis on not long.

So the original order is a good standard version, but it is not the only possible one.

Russian has no articles like English a/an and the.

So театр by itself can mean:

• a theater
• the theater

The exact meaning depends on context.

In this sentence, English usually uses the theater because the entrance to the theater sounds like a specific, identifiable place. But Russian does not need a separate word for that.

The main word stress is:

• Я
• недОлго
• ждАл
• у вхОда
• в теАтр

A rough pronunciation guide:

• недОлго ≈ nye-DOL-ga
• ждал ≈ zhdal
• вхОда ≈ FHO-da
• теАтр ≈ tee-ATr

One tricky point for English speakers is театр: it has two clear vowel parts, and the stress is on А.

So the full sentence is pronounced approximately:

ya nye-DOL-ga zhdal oo FHO-da f tee-ATr

That is only approximate, but it helps with the stress pattern.