Learn Hardcore Russian: Положи матрас на кровать, пожалуйста.

Положи is the imperative (command/request) form of the verb положить (perfective).
It’s the 2nd person singular imperative, used when you’re speaking informally to one person (to a friend, family member, child, etc.): (you) put / place (it).

What’s the difference between положить and класть (or положи and клади)?

They’re a common aspect pair:

класть / клади = imperfective (“to be putting / to put repeatedly / the process”)
положить / положи = perfective (“to put once, achieve the result”)

In a simple one-time instruction like this, Russian often prefers the perfective imperative: положи = “put it (so that it ends up there).”

Why is it Положи, not Ложи? I’ve heard people say ложить.

In standard Russian, the correct verb is положить (perfective) and класть (imperfective).
Forms like ложить / ложи are widely heard in casual speech in some regions, but they’re considered nonstandard and are best avoided in learning and formal contexts.

What case is матрас, and why doesn’t it change?

Матрас is the direct object, so it’s in the accusative case.
For many inanimate masculine nouns, the accusative equals the nominative, so it stays матрас (not матраса).

Why is it на кровать (not на кровати)?

Because на changes meaning depending on the case:

на + accusative = motion / destination (onto): на кровать = “onto the bed”
на + prepositional = location (on): на кровати = “on the bed”

Here it’s “put it (so it ends up) onto the bed,” so на кровать is used.

Why does кровать look the same in the accusative?

Кровать is a feminine noun ending in a soft sign (ь).
For many feminine soft-sign nouns, the accusative singular is identical to the nominative singular:

nominative: кровать
accusative: кровать

So it doesn’t visibly change here.

Is the comma before пожалуйста necessary?

Yes, typically пожалуйста is set off with commas because it functions like a parenthetical politeness marker:

Положи матрас на кровать, пожалуйста.

You can also place it elsewhere, usually with commas:

Положи, пожалуйста, матрас на кровать.
Пожалуйста, положи матрас на кровать.

How would I say this more formally or politely to someone I don’t know well?

Use the plural/formal imperative:

Положите матрас на кровать, пожалуйста.

That’s the standard polite version to one person (formal you) or to multiple people.

Can Russian change the word order here, and does it change the meaning?

Yes, word order is flexible, and the core meaning stays the same. Changes usually add emphasis:

Положи матрас на кровать, пожалуйста. (neutral)
Матрас положи на кровать, пожалуйста. (emphasis on матрас—the mattress, not something else)
На кровать положи матрас, пожалуйста. (emphasis on destination—onto the bed)

How is this sentence pronounced (stress)?

Common stress pattern:

положи́ (stress on the last syllable)
матра́с
крова́ть

So: Положи́ матра́с на крова́ть, пожа́луйста.