Proszę zachować się ciszej podczas lekcji.

Here proszę is used to make the sentence polite, like please in English.

In this kind of school, office, or public instruction, proszę + infinitive is very common:

• Proszę usiąść. = Please sit down.
• Proszę poczekać. = Please wait.
• Proszę zachować się ciszej. = Please behave more quietly / please be quieter.

So it is not the full meaning of I ask here, even though historically it comes from that verb. In this sentence, it functions as a polite request.

Because after proszę in this structure, Polish usually uses the infinitive.

So the pattern is:

• proszę + infinitive

Examples:

• Proszę mówić wolniej. = Please speak more slowly.
• Proszę zamknąć drzwi. = Please close the door.
• Proszę zachować się ciszej. = Please behave more quietly.

This is one of the standard ways to make polite requests in Polish.

Zachować się means to behave or to conduct oneself.

It is a reflexive verb phrase:

• zachować się = to behave
• zachowywać się = to be behaving / to behave habitually

Examples:

• On źle się zachowuje. = He is behaving badly.
• Proszę się dobrze zachować. = Please behave properly.

So in your sentence, zachować się ciszej means something like behave more quietly or be quieter in your behavior.

Because zachować się is the normal dictionary form of this verb meaning to behave. The się is part of the verb.

This is not a word-for-word equivalent of English oneself in a way that you can always remove. If you remove się, the meaning changes.

Compare:

• zachować się = to behave
• zachować coś = to preserve / keep / retain something

For example:

• zachować spokój = keep calm
• zachować się spokojnie = behave calmly

So się is essential here.

Ciszej is the comparative form, meaning more quietly or quieter.

• cicho = quietly / silent
• ciszej = more quietly / quieter

In Polish, polite requests often use the comparative in situations where English also says quieter, even if English might sometimes simply say be quiet.

So:

• Proszę mówić ciszej. = Please speak more quietly.
• Proszę zachować się ciszej. = Please behave more quietly.

If you said cicho, that would not fit as naturally in this sentence.

It comes from cicho.

The comparison is:

• cicho = quietly
• ciszej = more quietly

This is an irregular-looking comparative, so it is best learned as a form on its own.

Related forms:

• cichy = quiet
• ciszej = quieter / more quietly

Polish often uses the same comparative form for both adjective and adverb meanings, depending on context.

Because podczas requires the genitive case.

The noun lekcja is feminine, and its genitive singular form is lekcji.

So:

• lekcja = lesson / class
• podczas lekcji = during the lesson / during class

This is a fixed grammar rule:

• podczas obiadu = during dinner
• podczas spotkania = during the meeting
• podczas lekcji = during class

Yes, often you can, but the nuance is a little different.

• podczas lekcji = during the lesson / during class
• na lekcji = in class / at class

In many contexts both are natural:

• Proszę zachować się ciszej podczas lekcji.
• Proszę zachować się ciszej na lekcji.

Podczas lekcji sounds a bit more like during the lesson period, while na lekcji is the very common everyday way to say in class.

Yes. It is polite and fairly standard, especially in schools, public notices, or instructions from a teacher.

It is less direct than a plain imperative.

Compare:

• Zachowujcie się ciszej podczas lekcji. = Be quieter during class.
  This is more direct, addressed to more than one person.
• Proszę zachować się ciszej podczas lekcji. = Please behave more quietly during class.
  This is more polite and softer.

Teachers, staff, and officials often use proszę + infinitive because it sounds polite but still authoritative.

This is about aspect.

• zachować się = perfective
• zachowywać się = imperfective

Very roughly:

• perfective often presents the action as a whole, or as something to do now / achieve
• imperfective focuses more on ongoing, repeated, or habitual action

In requests, both can appear, but they can sound slightly different:

• Proszę zachować się ciszej.
  Please be quieter now / please behave more quietly.
• Proszę zachowywać się ciszej.
  Please keep behaving more quietly / please behave more quietly in an ongoing way.

In a classroom instruction, zachować się is very natural.

The given word order is completely natural:

• Proszę zachować się ciszej podczas lekcji.

Polish word order is more flexible than English, so other versions are possible, for example:

• Podczas lekcji proszę zachować się ciszej.

This puts more emphasis on during class.

But for a learner, the original order is a very good standard pattern:

• proszę + infinitive + adverb + time/place phrase

This structure does not clearly show number by itself, so it can be used to address:

• one person politely
• several people
• a general audience

That is one reason why it is so useful in signs, announcements, and classroom instructions.

If you wanted to be more specific with an imperative:

• Zachowuj się ciszej. = Be quieter. (to one person, informal)
• Zachowujcie się ciszej. = Be quieter. (to several people, informal/plural)

But Proszę zachować się ciszej works very well in a general, polite way.