Я слила макароны через дуршлаг и сразу подала их на стол.

Because the speaker is female.

In the Russian past tense, verbs agree with the subject in gender and number:

• я слила, я подала = a woman is speaking
• я слил, я подал = a man is speaking
• мы слили, мы подали = plural

Unlike English, Russian past-tense verbs do not show person the same way. The form tells you gender/number, and the pronoun я tells you it means I.

They describe completed, one-time actions in sequence:

1. she drained the pasta
2. she served it right away

Perfective verbs are very common for this kind of narrative chain of finished events.

Here:

• слить = to drain off, to pour off completely
• подать = to serve, to bring out

If you used imperfective forms such as сливала or подавала, the meaning would sound more like a process, repetition, or background action rather than two completed steps.

Yes. In everyday Russian, макароны is very often used as a plural noun for pasta or macaroni as food.

So Russian treats it grammatically as plural:

• макароны готовы
• я люблю макароны
• слила макароны
• подала их

That is why the pronoun is also plural: их = them.

Literally, yes, it is the water that is being poured off. But in Russian, just like in English, people often speak more naturally and economically.

So слила макароны means something like:

• drained the pasta
• poured the pasta into a colander and got rid of the water

A more explicit version would be:

• слила воду с макарон = drained the water off the pasta

But the shorter version is completely normal in context.

Через normally takes the accusative case, and it often means through.

So:

• через дуршлаг = through a colander

Here дуршлаг is masculine, inanimate, singular. For many inanimate masculine nouns, the accusative form is the same as the nominative, so you see:

• nominative: дуршлаг
• accusative: дуршлаг

That is why the form does not change.

Sometimes yes, but it would express a slightly different image.

• через дуршлаг focuses on the idea of passing the pasta/water through the colander
• в дуршлаг focuses on putting or throwing it into the colander

So with this sentence, через дуршлаг is very natural because the key idea is the draining process.

You may also hear another common expression:

• откинуть макароны на дуршлаг = to dump/strain the pasta into a colander

Not exactly. Подать на стол is an idiomatic expression meaning:

• to serve
• to put on the table for eating
• to bring out as food

So подала их на стол means served them.

It is not just about physical placement. It implies presenting the food for the meal.

Not always, but it is very natural here.

• подала их на стол = served them
• подала на стол = served up / put on the table

Russian can sometimes omit the object if it is obvious from context. But keeping их makes the sentence clearer and more complete: it tells you explicitly that what she served was the pasta.

Because the same subject continues for both verbs.

Russian, like English, often avoids repeating the subject when two actions have the same doer:

• Я слила макароны и сразу подала их на стол.

Repeating я would usually sound unnecessary:

• Я слила макароны и я сразу подала их на стол.

That is possible for special emphasis, but it is not the neutral version.

Сразу means right away, immediately, or at once.

In this sentence it shows that the second action happened without delay:

• she drained the pasta and immediately served it

Russian word order is flexible, so you could move сразу for slightly different rhythm or emphasis, for example:

• Я слила макароны и сразу подала их на стол.
• Я сразу слила макароны и подала их на стол.
• Я слила макароны и подала их сразу на стол.

The original version is the most natural if you want immediately to relate to the serving step.

A quick breakdown:

• Я — nominative, the subject
• макароны — accusative plural, direct object of слила
  For inanimate plural nouns, accusative often looks the same as nominative.
• через дуршлаг — дуршлаг is accusative after через
• их — accusative plural pronoun, direct object of подала
• на стол — стол is accusative after на, because it expresses direction/result: serving onto/to the table

So the sentence combines a subject, two completed actions, and several objects/phrases in very standard Russian patterns.