Learn Hardcore Russian: Ты должен повторять слова каждый день.

Должен is personal and stronger, closer to must / have to.
Structure: [person] + должен + infinitive
Example: Ты должен повторять слова каждый день.
This sounds like a clear rule or duty directed at you.
Надо / нужно are impersonal and often sound a bit softer, closer to need to / should.
Structure: (кому?) + надо / нужно + infinitive
Example:
- Тебе нужно повторять слова каждый день.
- Надо повторять слова каждый день. (general advice, no specific person)

So the choice of должен emphasizes that the obligation is clearly attached to ты and may sound a bit stricter or more categorical than нужно or надо.

What does должен agree with, and how would the sentence change for a woman or for several people?

Grammatically, должен is a short-form adjective. It must agree with the subject in:

Number (singular/plural)
Gender (masculine/feminine/neuter in the singular)

In the given sentence, ты refers to a male, so we use the masculine form должен.

To a man (informal singular):
Ты должен повторять слова каждый день.
To a woman (informal singular):
Ты должна повторять слова каждый день.
(должна – feminine singular)
To several people or polite вы (plural):
Вы должны повторять слова каждый день.
(должны – plural, used for both masculine and feminine groups, and also for polite singular вы)
About yourself (we):
Мы должны повторять слова каждый день.

So the form of должен (должен / должна / должно / должны) changes to match who has the obligation.

Why is повторять in the infinitive, and what exactly is this form?

Повторять is the infinitive form of the verb, roughly equivalent to English to repeat.

In Russian, after words of obligation or necessity like должен, надо, нужно, можно, you normally use the infinitive:

Он должен работать. – He must work.
Мне нужно учить слова. – I need to learn words.
Ты можешь повторять слова. – You can repeat the words.

So in Ты должен повторять слова каждый день, the pattern is:

должен (obligation) + повторять (infinitive: the action you must do)

Why do we use imperfective повторять instead of perfective повторить?

Russian aspect is very important here:

Повторять – imperfective, focuses on the process, habit, or repeated action.
Повторить – perfective, focuses on a single complete act of repeating.

The phrase каждый день (every day) clearly describes a regular, repeated action. For repeated or habitual actions, Russian normally uses the imperfective:

Ты должен повторять слова каждый день.
You are supposed to do this regularly, as a routine.

If you use повторить, it sounds like a single event:

Ты должен повторить слова завтра.
You must repeat the words (once) tomorrow.

So повторять matches каждый день because it describes an ongoing habit, not one isolated act.

What case is слова, and why is it not слов?

The noun слово (word) declines like this in the plural:

Nominative plural: слова
Accusative plural (inanimate): слова
Genitive plural: слов

In the sentence Ты должен повторять слова каждый день, слова is the direct object of повторять, so it must be in the accusative case.

Because слово is inanimate, the accusative plural is identical to the nominative plural: слова.

The form слов is genitive plural. You use it in different contexts, for example:

After certain numbers: много слов (many words)
With negatives: у меня нет слов (I have no words)
After some prepositions: из слов, без слов

Here we just need a simple direct object, so the correct form is слова, not слов.

Could we move каждый день to another position, like Каждый день ты должен повторять слова? Does the meaning change?

Yes, Russian word order is flexible, and several versions are grammatically correct:

Ты должен повторять слова каждый день.
Каждый день ты должен повторять слова.
Ты каждый день должен повторять слова.

The basic meaning stays the same: you must repeat the words every day.

The differences are mostly about emphasis and rhythm:

Каждый день ты должен повторять слова.
Slightly stronger focus on каждый день – the daily nature of the action is highlighted first, like:
Every day, you must…
Ты каждый день должен повторять слова.
Emphasis on the combination ты … должен and that this is your daily duty.
Ты должен повторять слова каждый день.
A very natural, neutral word order; many textbooks present this pattern.

All of these sound natural in everyday speech.

Is ты должен very strong or rude? How can I say this more politely or softly?

Ты должен can sound quite strong, like you must, especially if said with a firm tone. It is not automatically rude, but it is direct and can feel bossy depending on context.

Softer or more polite alternatives:

Тебе нужно повторять слова каждый день.
More like you need to; sounds gentler.
Тебе надо повторять слова каждый день.
Similar to нужно, often used in everyday speech.
To a person you address as вы (formal or plural):
- Вам нужно повторять слова каждый день. (polite, softer)
- Вы должны повторять слова каждый день. (more direct, like you must)

So, if you are a teacher and want to sound less strict, Вам нужно повторять слова каждый день or Нужно повторять слова каждый день will usually feel more polite than Вы должны…

Does должен have a future tense? How do you talk about future obligations with должен?

Должен itself does not have a separate future tense form. It is a short-form adjective used in the present or past:

Present obligation:
Я должен работать. – I must work / I have to work.
Past obligation:
Я должен был работать. – I had to work.
(должен был / должна была / должны были – past forms)

For future obligations, Russian usually still uses должен in the same form and relies on context, time expressions, or the infinitive:

Завтра ты должен повторять слова.
Tomorrow you must repeat the words.

You can also use other verbs to emphasize future necessity:

Тебе придётся повторять слова каждый день.
You will have to repeat the words every day.

But you do not form a future tense of должен like a normal verb. You keep должен and add time markers or change the rest of the sentence.

Can we omit ты and just say Должен повторять слова каждый день?

In Russian, subject pronouns (like я, ты, он) are often omitted because the verb ending usually shows the person. But with должен, things are trickier:

Должен повторять слова каждый день.
This sounds incomplete or like a note where the subject is understood from context (for example, written on a study plan only for you).

Normally, in clear, neutral sentences, you would keep the pronoun:

Ты должен повторять слова каждый день.

If you want a general, impersonal statement with no specific person, you usually switch to надо / нужно:

Надо повторять слова каждый день.
Нужно повторять слова каждый день.

So yes, omitting ты is possible in some contexts, but for clear, standard sentences, Ты должен… is better.

How would I say this to a group of students in a classroom, using polite вы?

To address several students (or one student formally) you use вы and the plural form должны:

Вы должны повторять слова каждый день.

This means:

Either: You all (as a group) must repeat the words every day.
Or (formal singular): You (sir/ma’am) must repeat the words every day.

If you want to sound less strict and more like giving advice:

Вам нужно повторять слова каждый день.
Вам надо повторять слова каждый день.

How is each word in Ты должен повторять слова каждый день pronounced, and where is the stress?

Here is a rough pronunciation guide with stressed syllables in capitals (Latin transcription is approximate):

Ты – ty (like English t
- short i as in tip)
должен – DOL-zhen
- DOHL like doll but with a darker L
- zhen like zh in measure
  - en
повторять – pav-ta-RYAT’
- pa as in palm but shorter
- vta merges quickly: vtah
- RYAT’ – stress here; ryat with soft t’ at the end
слова – sla-VA
- sla like slah
- VA stressed, like vah
каждый – KAZH-dy
- KAZH – like cash but with zh sound at the end
- dy – short, soft dee
день – den’
- like den but with a soft n’ at the end

Stress pattern in the whole sentence:
Ты DOLжен повтоРЯТЬ слоВА КАЖдый ДЕНЬ.