Я должен перед сном выключить телефон.

I have to turn off my phone before going to sleep.

Breakdown of Я должен перед сном выключить телефон.

• я: I
• телефон: the phone
• перед: before
• выключить: to turn off
• сон: the sleep
• должен: must / to be supposed to

Questions & Answers about Я должен перед сном выключить телефон.

Why is должен used here, and why not должен(а) / должны?

Должен is a short-form adjective meaning obliged / supposed to / have to. It agrees with the subject in gender and number:

• Я должен (male speaker)
• Я должна (female speaker)
• Мы должны (we)
• Он должен / она должна / они должны

So the form depends on who is speaking, not on the verb выключить.

Do I need to say мне (as in мне нужно) with я должен?

No. With я должен you normally do not add мне because я is already the subject who has the obligation.
Compare:

• Я должен выключить телефон. = I must / I’m supposed to…
• Мне нужно выключить телефон. = I need to… (literally to me it is necessary)

Both are common, but the grammar is different.

Why is the verb in the infinitive: выключить, not a conjugated form?

After должен / должна / должны, Russian typically uses an infinitive to name the required action:

• Я должен (что сделать?) выключить… It works like English must + base verb: must turn off, not must turns off.

What case is телефон, and why?

Телефон is in the accusative case because it is the direct object of выключить (to turn off what?):

• выключить телефон (turn off the phone)

For masculine inanimate nouns like телефон, the accusative looks the same as the nominative: телефон.

Why is it перед сном and not перед сон?

The preposition перед requires the instrumental case.
So сон becomes сном (instrumental singular):

• перед сном = before sleep / before going to bed

This is a fixed, very common phrase.

Could I also say до сна instead of перед сном?

Sometimes, but the meaning is a bit different:

• перед сном = right before sleeping / at bedtime (very natural here)
• до сна = before sleep in a more general “earlier than sleeping” sense; it can sound less idiomatic in this exact sentence

For a bedtime habit, перед сном is usually the best choice.

Why is выключить (perfective) used instead of выключать (imperfective)?

Выключить is perfective, focusing on the completed result: the phone ends up off. That fits well with obligations: you must do it (and finish it).

If you say выключать (imperfective), it tends to describe a repeated habit/process:

• Я должен перед сном выключать телефон. = I’m supposed to (habitually) turn off my phone before bed.

Both can be correct; выключить sounds like a concrete required action (often “tonight” or as a rule stated as a single action).

Is the word order fixed? Can I move перед сном?

Word order is flexible. All of these are possible, with slightly different emphasis:

• Я должен перед сном выключить телефон. (neutral; sets the time first)
• Я должен выключить телефон перед сном. (emphasizes the action first)
• Перед сном я должен выключить телефон. (strong focus on “before sleep”)

Russian uses word order more for emphasis than for basic grammar.

Does должен mean “must” (strong obligation) or “should” (advice)?

Должен often corresponds to must / have to (a real obligation), but context can soften it toward should.
If you want clearly “advice,” common alternatives are:

• Мне надо… / Мне нужно… (I need to / I should…)
• Мне следует… (I ought to…; more formal)

Still, я должен is very common and not necessarily harsh—tone and context matter.

How do I pronounce this sentence—where are the stresses?

Typical stresses:

• Я до́лжен
• пе́ред сно́м
• вы́ключить телефо́н

So: Ya DOHZhen PEred SNOM VYklyuchit teleFON.