Learn Hardcore Russian: Я положил зарядку в сумку, чтобы не забыть её. - I put the charger in my bag so as not to forget it.

Положил is the past tense, masculine, perfective form of the verb положить (to put/place as a completed action). In this sentence, the speaker is describing a finished action: I put it in the bag (and it’s done).

класть/кладу is the imperfective verb (to be putting / to put repeatedly / the process). You’d use it for an ongoing or habitual action, e.g. Я кладу зарядку в сумку каждый день (I put my charger in my bag every day).
In the past, imperfective would be клал: Я клал зарядку в сумку (I was putting / used to put), but положил is the normal choice for a single completed action.

What does зарядку mean here—why is it not зарядное устройство?

Зарядка is a very common everyday word that can mean charger (device/cable/adapter), especially in casual speech. Зарядное устройство is more formal/technical (charging device).
So Я положил зарядку в сумку is like I put my charger in my bag (natural, colloquial).

Why is зарядку in the accusative case?

Because it’s the direct object of положил (put what?). For a feminine noun like зарядка, the accusative singular ends in -у:

nominative: зарядка
accusative: зарядку

Why is it в сумку (accusative) and not в сумке (prepositional)?

With в:

в + accusative = motion into a place (into the bag): положил в сумку
в + prepositional = location (in the bag): лежит в сумке (it’s lying in the bag)
Since the action involves movement of the charger into the bag, you use в сумку.

What’s the function of чтобы here?

Чтобы introduces a purpose clause: in order to / so that.
So …, чтобы не забыть её means …, so as not to forget it / in order not to forget it.

Why is it не забыть (infinitive) and not something like не забуду?

After чтобы, Russian often uses:

past tense in the main clause + чтобы + infinitive to express purpose in a general way: положил…, чтобы не забыть (I put…, so as not to forget)
You can also use a finite verb in some contexts: …, чтобы я не забыл (so that I wouldn’t forget), which sounds a bit more explicit and emphasizes the subject (I) inside the purpose clause. The infinitive version is very common and natural here.

Why is её used, and what exactly does it refer to?

её means her / it (accusative/genitive form). Here it refers to зарядку (a feminine noun), so English it becomes Russian её.
Russian pronouns must match the grammatical gender of the noun, not “real-life” gender.

Can её be omitted?

Often yes, if it’s clear from context: Я положил зарядку в сумку, чтобы не забыть.
But including её is also natural, especially if you want to make the reference explicit or avoid any ambiguity about what you don’t want to forget.

Is there any difference between забыть её and не забыть её?

Yes:

забыть её = to forget it
не забыть её = not to forget it / to remember it (but phrased as avoid forgetting)
With чтобы, the negative form is common when the goal is preventing something: I put it there so I wouldn’t forget it.

What aspect is забыть, and could it be забывать?

забыть is perfective (to forget as a completed event: forget it (once)). That matches the idea of a single possible mistake you want to avoid.
забывать is imperfective (to forget repeatedly / as a habit / in general). You might use it if you’re talking about a recurring problem:

Я кладу зарядку в сумку, чтобы не забывать её дома. (…so I don’t keep forgetting it at home.)

Why is the subject я not written?

Russian often drops subject pronouns when the verb ending already shows the person/number, or when it’s obvious from context. положил already implies I (male speaker) in past tense.
You can add я for emphasis or contrast: Я положил зарядку… (I put it in the bag [not someone else]).

What changes if the speaker is female?

Past tense agrees in gender with the speaker (when I is the subject in past tense):

male: Я положил…
female: Я положила…
Everything else can stay the same: Я положила зарядку в сумку, чтобы не забыть её.

Could I say в мою сумку instead of в сумку?

Yes. в сумку is just more general and often implies my bag from context. If you want to be explicit:

в мою сумку (into my bag)
Or if it’s someone else’s bag: в её сумку, в его сумку, etc.

Is положить interchangeable with поставить here?

Not always.

положить = put something down (often implying laying it) or just “place” generally
поставить = put something in an upright/standing position (set it down “standing”)
For a charger, положить is the default because it’s not something you normally “stand up.”

Does word order matter here? Can I move things around?

Russian word order is flexible, but it changes emphasis. The neutral version is exactly what you have. For example:

Чтобы не забыть её, я положил зарядку в сумку. (emphasis on the purpose)
Я положил в сумку зарядку, чтобы не забыть её. (slightly emphasizes the location в сумку)
All are grammatical; choose based on what you want to highlight.

What’s the difference between забыть её and забыть про неё?

забыть её = forget it (fail to take it / leave it behind)
забыть про неё = forget about it (stop thinking about it / overlook it mentally)
In this context (not leaving the charger behind), забыть её is the most direct and common.