Learn Hardcore Russian: Не забудь взять зарядку с собой. - Don't forget to take the charger with you.

It’s the second-person singular imperative of the perfective verb забыть “to forget.” With не, it means “Don’t forget.” The polite/plural version is Не забудьте. The soft sign ь at the end is the typical imperative marker for many verbs in the ты-form.

Why is the perfective забыть used (not the imperfective забывать)?

Perfective in the imperative is used for a one-time, goal-oriented result: “make sure you don’t end up forgetting (this time).”

Не забудь… = a specific reminder for this occasion.
Не забывай… (imperfective) = “Don’t forget (habitually),” a standing instruction or repeated reminder.

Why is взять (perfective infinitive) used instead of брать?

Because the action is single and goal-oriented: “to take (and have it with you) on this specific occasion.”

Не забудь взять… = “Don’t forget to take (it this time).”
For habitual advice, you’d switch both verbs to imperfective: Не забывай брать зарядку = “Don’t forget to take a charger (as a habit).”

Why is it взять (infinitive) and not the imperative возьми?

After Не забудь, Russian uses an infinitive to express what not to forget to do: Не забудь + infinitive (here, взять).
Saying Не забудь возьми is ungrammatical. If you wanted two separate imperatives, you’d split them: Возьми зарядку. Не забудь. (two sentences).

What case is зарядку, and why?

Зарядку is accusative singular, the direct object of взять. The base noun зарядка (feminine) changes to зарядку in the accusative. Stress: за-ря́д-ку (stress on я).

Does зарядка mean “charger” or “exercise”?

Both exist in Russian:

In this context it means “charger” (phone/laptop charger). Very common in everyday speech.
It can also mean “morning exercise/warm-up.” To be unambiguous or more formal, you can say зарядное устройство; colloquially you may also hear зарядник. You can specify: зарядка для телефона.

What does с собой literally mean, and how is it used?

Literally “with oneself.” It’s the preposition с + instrumental of the reflexive pronoun (собой from себя).
With verbs like брать/взять, с собой means “to take along (with you/with me/etc.)”:

Возьми с собой воду. = “Take water with you.”
Мы взяли с собой пледы. = “We took blankets with us.”

Can I say с тобой instead of с собой here?

No. The fixed, idiomatic pattern with “take along” is взять/брать с собой.

Correct: Возьми это с собой.
Not standard in this structure: Возьми это с тобой.
Do use с тобой for “go/be with you”: Я пойду с тобой = “I’ll go with you.”
Note: Возьми меня с собой = “Take me with you” (the “with you” still uses с собой).

Is the word order fixed? Can I say взять с собой зарядку instead of взять зарядку с собой?

Both are fine and natural:

взять с собой зарядку (very common)
взять зарядку с собой (also fine) You can also front the object for emphasis: Не забудь зарядку взять с собой, which highlights “the charger.”

Can I drop взять or с собой?

Не забудь зарядку is common and usually understood as “Don’t forget the charger (to bring it).” It names the thing not to forget, leaving the action implied.
Не забудь взять зарядку explicitly mentions the action “to take.”
Не забудь взять зарядку с собой makes it crystal clear you should bring it with you.
Avoid: Не забудь зарядку с собой (ungrammatical because с собой needs a verb like взять/брать).

How do I make this polite or address multiple people?

Use the вы-form imperative: Не забудьте взять зарядку с собой.
To soften, add пожалуйста (with commas): Не забудьте, пожалуйста, взять зарядку с собой.
Singular informal: Не забудь, пожалуйста, взять зарядку с собой.

I heard perfective negatives in commands can be odd. Is Не забудь really okay?

Yes. While many negative prohibitions use the imperfective (Не читай! “Don’t read!”), certain one-off, “endpoint” actions regularly take the perfective in negative imperatives:

Не забудь (don’t end up forgetting)
Не опоздай (don’t end up being late)
Не потеряй (don’t end up losing it)
These are standard and idiomatic.

How is the sentence pronounced and where is the stress?

Stresses: Не забу́дь взя́ть заря́дку с собо́й.
A simple transliteration: Ne zabud’ vzyat’ zaryadku s soboy.
Tips:

The double “s” in с собой is pronounced like a long
The final -ть in забудь/взять softens the consonant ([tʲ]).

Are there natural synonyms for “take” here?

Yes. Colloquial alternatives include:

Не забудь прихватить зарядку (с собой). = “Don’t forget to grab the charger (to take with you).”
Не забудь захватить зарядку (с собой). Similar nuance: “grab/take along.”
These are informal but very common.