Он поднял тяжёлый мешок.

Мешок is in the accusative singular because it is the direct object of the verb поднял.

In other words, it is the thing that he lifted.

A very common beginner point is that мешок looks the same as the nominative form. That is normal: for masculine inanimate nouns, the accusative singular is usually identical to the nominative singular.

• nominative: мешок
• accusative: мешок

So even though the form does not change, the function in the sentence does.

Because тяжёлый agrees with мешок, and here мешок is masculine singular inanimate.

For masculine inanimate nouns, the accusative singular adjective form is the same as the nominative singular form. So:

• nominative: тяжёлый мешок
• accusative: поднял тяжёлый мешок

That may feel strange if you expect the object to always look different, but in Russian it often does not for masculine inanimate nouns.

Compare with a feminine noun, where the change is clearer:

• тяжёлая сумка
• поднял тяжёлую сумку

Because the subject is он, meaning he.

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

• он поднял = he lifted
• она подняла = she lifted
• оно подняло = it lifted
• они подняли = they lifted

So поднял is the masculine singular past form.

The infinitive is поднять.

The past tense is formed from the verb stem plus -л:

• поднять → поднял (masculine)
• поднять → подняла (feminine)
• поднять → подняло (neuter)
• поднять → подняли (plural)

This is a very common past-tense pattern in Russian.

Also, поднять is the perfective member of the pair:

• поднимать = imperfective
• поднять = perfective

Because поднял is perfective, and it presents the action as a completed whole: he lifted the sack.

The imperfective поднимал would suggest something different, depending on context:

• he was lifting it
• he used to lift it
• he tried lifting it
• he was in the process of lifting it

So:

• Он поднял тяжёлый мешок. = He lifted the heavy sack.
• Он поднимал тяжёлый мешок. = He was lifting / used to lift / was trying to lift the heavy sack.

For a single completed action, поднял is the normal choice.

Yes, often it can.

Russian frequently omits subject pronouns when the subject is already clear from context. So you might also hear:

• Поднял тяжёлый мешок.

However, keeping он can be natural if:

• you are introducing the subject,
• you are contrasting him with someone else,
• or you want a slightly clearer or more explicit sentence.

So both are possible, but Он поднял тяжёлый мешок is a perfectly normal full sentence.

Because Russian has no articles.

Russian does not have direct equivalents of a/an and the as normal articles. Whether the meaning is a heavy sack or the heavy sack is understood from context.

So тяжёлый мешок can mean:

• a heavy sack
• the heavy sack

depending on the situation.

This is one of the biggest differences from English, and learners usually just have to get used to letting context do that job.

No, Russian word order is fairly flexible.

Он поднял тяжёлый мешок is a neutral, straightforward order:

• subject
• verb
• object

But Russian can move things around for emphasis, contrast, or style. For example:

• Тяжёлый мешок он поднял.
  Emphasizes the heavy sack.

• Он тяжёлый мешок поднял.
  Also possible, often with a particular spoken emphasis.

Even though many orders are possible, the sentence you were given is the most neutral and beginner-friendly one.

Ё is very important here.

In тяжёлый, the ё is always stressed and sounds roughly like yo.

A rough pronunciation guide:

• тяжёлый ≈ tya-ZHO-lyy
• поднял ≈ pad-NYAL
• мешок ≈ mi-SHOK

A few useful points:

• ё always carries stress
• ж is like the s in measure
• ш is like sh in shop

So the stress pattern is:

• подня́л
• тяжёлый
• мешо́к

Not always.

In textbooks and learner materials, ё is usually written clearly: тяжёлый. But in many native texts, people often write е instead, so you may see:

• тяжелый

Even when it is written with е, the word is still pronounced тяжёлый if that is the correct word.

This can be annoying for learners, because ё gives useful pronunciation information. So when studying, it is a good idea to learn words with ё where appropriate.

Because that is the normal neutral position for an attributive adjective in Russian: adjective + noun.

So:

• тяжёлый мешок = a heavy sack

Putting the adjective after the noun is much less neutral and often changes the feel:

• мешок тяжёлый usually sounds more like the sack is heavy
• or it creates a special emphasis/style effect

So in an ordinary noun phrase, тяжёлый мешок is the expected order.