Поклади рюкзак під стілець, будь ласка.

Поклади means put or place here.

It is the imperative form of the verb покласти, which is a perfective verb meaning to put / to place as a completed action. So поклади is used when telling one person to do it:

• Поклади рюкзак під стілець. = Put the backpack under the chair.

If you are speaking formally or to more than one person, you would use покладіть instead.

This is mainly about aspect, which is very important in Ukrainian.

• класти / клади = imperfective
• покласти / поклади = perfective

In a one-time command like Put the backpack under the chair, Ukrainian often prefers the perfective imperative, because the focus is on completing the action.

So:

• Поклади рюкзак... = put it there now, as a completed action
• Клади рюкзак... can sound more like go ahead and be putting it / put it down and may feel more process-oriented, repeated, or conversational depending on context

For a simple request like this, поклади is very natural.

Ukrainian often omits subject pronouns when they are clear from the verb form.

In поклади, the ending already shows that this is a command to one person. So adding ти is usually unnecessary.

• Поклади рюкзак під стілець. = normal
• Ти поклади рюкзак під стілець. = possible, but more emphatic or contrastive

English usually needs you less often in commands too, so this is similar in spirit.

Рюкзак is the direct object, so it is in the accusative case.

But рюкзак is a masculine inanimate noun, and for many masculine inanimate nouns, the accusative singular looks exactly like the nominative singular.

So:

• nominative: рюкзак
• accusative: рюкзак

That is why it appears unchanged in the sentence.

Because під can take different cases depending on the meaning:

• accusative = motion toward a place
• instrumental = location in a place

Here the backpack is being moved to a position under the chair, so Ukrainian uses the accusative:

• під стілець = to under the chair / under the chair as a destination

Compare:

• Поклади рюкзак під стілець. = Put the backpack under the chair.
• Рюкзак лежить під стільцем. = The backpack is under the chair.

So this is a very important pattern:

• motion -> під + accusative
• location -> під + instrumental

It is in the accusative singular after під, because the sentence describes movement to a place.

However, стілець is another masculine inanimate noun, so its accusative form is the same as its nominative form:

• nominative: стілець
• accusative: стілець

That is why you do not see a different ending here.

The instrumental form is different:

• під стільцем = under the chair

Будь ласка means please in this sentence.

It is a very common polite expression in Ukrainian. You will also hear it used for you're welcome, depending on context.

Here:

• Поклади рюкзак під стілець, будь ласка. = Put the backpack under the chair, please.

You do not need to analyze it literally when learning everyday speech; it is best treated as a fixed polite phrase.

Because будь ласка is treated as a parenthetical polite expression, so it is often separated by commas in writing.

That is why you get:

• Поклади рюкзак під стілець, будь ласка.

You may also see:

• Будь ласка, поклади рюкзак під стілець.

Both are normal. The comma helps show that будь ласка is not part of the main grammatical structure of the command.

Use the plural/formal imperative:

• Покладіть рюкзак під стілець, будь ласка.

This can mean:

• speaking to several people
• speaking to one person formally

So:

• поклади = singular informal
• покладіть = plural or formal

This is similar to the tu / vous distinction in some other languages, although English does not mark it directly.

Yes. Ukrainian word order is fairly flexible, because case endings help show what each word is doing.

The most neutral order here is:

• Поклади рюкзак під стілець, будь ласка.

But other orders are possible for emphasis:

• Будь ласка, поклади рюкзак під стілець.
• Рюкзак поклади під стілець, будь ласка.
  This emphasizes the backpack.
• Під стілець поклади рюкзак, будь ласка.
  This emphasizes under the chair.

So changing the order usually changes focus or emphasis, not the basic meaning.

Yes, you might hear that, but there is a nuance.

• поклади comes from покласти and suggests laying/putting
• постав comes from поставити and suggests setting/standing

With some objects, either verb can work depending on how the object is positioned. A backpack can sometimes be set upright or put/laid somewhere, so both may be possible in real life.

Still, поклади рюкзак під стілець is completely natural and safe.

A helpful stress guide is:

• поклади́
• рюкза́к
• під
• сті́лець
• будь ла́ска

A rough English-style approximation is:

pokla-DY ryuk-ZAK pid STEE-lets, bud LA-ska

A few notes:

• рю in рюкзак is like ryu
• і is like the vowel in machine
• ць at the end of стілець is a soft ts
• дь in будь is soft as well

If you want to sound natural, the stress pattern is especially important.