Bitte lass die Beeren im Kühlschrank auftauen.

Because lass is the informal singular imperative of lassen.

• lassen = the infinitive, meaning to let / to leave
• du lässt = you let / you leave
• lass! = let! / leave!

So Bitte lass die Beeren im Kühlschrank auftauen. is said to one person in an informal way.

Other versions would be:

• Lasst die Beeren im Kühlschrank auftauen. = speaking to more than one person informally
• Lassen Sie die Beeren im Kühlschrank auftauen. = speaking formally

Here lassen means something like let or leave.

So die Beeren im Kühlschrank auftauen lassen means:

• to let the berries thaw in the fridge
• to leave the berries in the fridge to thaw

It does not mean that the berries are actively doing something on purpose. It is more like allowing the process to happen.

Because auftauen is an infinitive, and with lassen that infinitive usually goes to the end.

The basic structure is:

• lass + object + place + infinitive

So:

• lass = imperative verb
• die Beeren = object
• im Kühlschrank = location
• auftauen = infinitive at the end

This is very common in German:

• Lass ihn schlafen.
• Lass das Fenster offen stehen.
• Lass die Suppe abkühlen.

Die Beeren is in the accusative plural, because it is the direct object of lass.

You are telling someone to let the berries thaw, so the berries are the thing being affected by the action.

For plural nouns, die is the same in both nominative and accusative, so the form does not change:

• nominative plural: die Beeren
• accusative plural: die Beeren

Because this sentence describes location, not movement.

• im Kühlschrank = in the fridge → where the berries should thaw
• in den Kühlschrank = into the fridge → movement toward the inside of the fridge

German uses:

• dative for location: im Kühlschrank
• accusative for direction/movement: in den Kühlschrank

So here the berries are already considered to be in the fridge while thawing.

Im is a contraction of in dem.

• in dem Kühlschrank → im Kühlschrank

This is extremely common in German.

Other examples:

• im Haus = in dem Haus
• im Garten = in dem Garten

So im Kühlschrank literally means in the refrigerator.

Using lassen makes the sentence sound like let it happen / leave it to thaw.

Compare:

• Lass die Beeren im Kühlschrank auftauen.
  = Let the berries thaw in the fridge / Leave them in the fridge to thaw.

• Taue die Beeren im Kühlschrank auf.
  = Thaw the berries in the fridge.

Both can work, but lassen emphasizes that you are not actively thawing them. You are simply leaving them somewhere so the thawing happens.

No. Bitte is optional, but it makes the command sound more polite.

Compare:

• Bitte lass die Beeren im Kühlschrank auftauen. = polite request
• Lass die Beeren im Kühlschrank auftauen. = more direct command

In German, bitte is very often used with imperatives to soften them.

It is informal, because of lass.

You would say this to:

• a friend
• a family member
• a child
• someone you address as du

For formal Sie, you would say:

• Bitte lassen Sie die Beeren im Kühlschrank auftauen.

Yes. Auftauen is a separable verb:

• infinitive: auftauen
• present tense: Die Beeren tauen auf.
• imperative with a direct command: Taue die Beeren auf!

But in this sentence, auftauen stays together because it is used as an infinitive after lass:

• Bitte lass die Beeren im Kühlschrank auftauen.

So the verb is separable in general, but here it appears in its full infinitive form.

Yes. That would still be grammatical:

• Bitte lass die Beeren auftauen.

That simply means Please let the berries thaw.

Adding im Kühlschrank gives important extra information about where the thawing should happen. That matters because thawing at room temperature and thawing in the fridge are different instructions.

A useful way to see it is:

• Bitte = polite softener
• lass = imperative verb
• die Beeren = direct object
• im Kühlschrank = location
• auftauen = infinitive at the end

So the pattern is roughly:

Bitte + imperative verb + object + location + infinitive

This is a very natural German sentence pattern with lassen.