Если ты устал, подумай, стоит ли идти на дополнительную лекцию сегодня.

In Russian, the past form of some verbs can describe a resulting current state.

• устал literally comes from устать = to get tired
• but ты устал often means you are tired

So Russian uses the idea you became tired and are now in that state.

Also note the form changes with gender:

• ты устал — said to a man
• ты устала — said to a woman

Подумай is the singular informal imperative of подумать.

It means something like:

• think about it
• give it some thought
• consider it

Because it is from the perfective verb подумать, it suggests a single, completed act of thinking/considering, not an ongoing process.

Compare:

• думай — think, keep thinking
• подумай — think it over / give it some thought

Yes, ты can often be omitted in Russian because the verb form already shows the person.

So you could hear:

• Если устал, подумай...

But keeping ты makes the sentence clearer and a bit more explicit:

• Если ты устал, подумай...

Russian often includes pronouns for emphasis, contrast, or clarity, even though they are not always necessary.

Стоит ли means is it worth... ? or whether it is worth...

In this sentence:

• подумай, стоит ли идти...
• think about whether it’s worth going...

This is a very common Russian pattern:

• Стоит ли делать это? — Is it worth doing this?
• Я не знаю, стоит ли ехать. — I don’t know whether it’s worth going.

Ли is a particle used to form an indirect yes/no question.

A key rule: ли usually comes right after the word it refers to.

So:

• стоит ли идти = whether it is worth going

Not:

• ли стоит идти ❌

You will often see this pattern in sentences like:

• Я не знаю, придёт ли он. — I don’t know whether he will come.
• Спроси, можно ли войти. — Ask whether it is possible to come in.

Russian often uses this as an impersonal construction.

• стоит ли идти = whether it’s worth going
• стоит ли тебе идти = whether it’s worth your going / whether you should go

Both are possible.

The version without тебе sounds more general and natural in many contexts.
Adding тебе makes the sentence more explicitly about you.

So:

• подумай, стоит ли идти — think whether it’s worth going
• подумай, стоит ли тебе идти — think whether it’s worth your going

After expressions like стоит ли..., Russian often uses the infinitive in a fairly general sense.

• стоит ли идти = is it worth going
• стоит ли пойти is also possible, but it can sound a bit more focused on the single act/decision to go

So:

• идти here sounds natural and neutral
• пойти would put a little more emphasis on the one-time action of setting off

In many everyday contexts, both can work, but стоит ли идти is a very common phrasing.

Russian uses на with many events and activities when meaning to or for attendance at that event.

So:

• идти на лекцию — to go to a lecture
• идти на концерт — to go to a concert
• идти на урок — to go to a lesson/class

Because this expresses motion toward a destination/event, на takes the accusative case.

That is why:

• лекция → лекцию
• дополнительная → дополнительную

Both the adjective and noun change because the adjective must agree with the noun.

Дополнительную means additional, extra, or supplementary.

So дополнительная лекция is an extra lecture in addition to the usual ones.

It does not automatically mean optional, although in some contexts an extra lecture may also be optional.

They are both required.

• Если ты устал, ...
  The first comma separates the if-clause from the main clause.

• подумай, стоит ли идти...
  The second comma separates the main clause from the subordinate clause introduced by the indirect-question structure стоит ли...

So the structure is:

• Если ты устал, — if you are tired,
• подумай, — think,
• стоит ли идти... — whether it is worth going...

Yes. Russian word order is flexible, and moving сегодня changes the emphasis slightly.

Original:

• стоит ли идти на дополнительную лекцию сегодня

Also possible:

• стоит ли сегодня идти на дополнительную лекцию
• сегодня подумай, стоит ли идти на дополнительную лекцию
  (less neutral, more emphasis on today)

The original version is perfectly natural.
Putting сегодня closer to идти can make it feel more directly tied to the action of going.

You would use вы and the corresponding verb forms:

• Если вы устали, подумайте, стоит ли идти на дополнительную лекцию сегодня.

Changes:

• ты → вы
• устал → устали
• подумай → подумайте

This can mean either:

• speaking politely to one person, or
• speaking to several people

Yes, the past form in the если clause changes for gender:

• to a man: Если ты устал...
• to a woman: Если ты устала...

The rest of the sentence stays the same:

• Если ты устала, подумай, стоит ли идти на дополнительную лекцию сегодня.