Я машинально положил ключи на полку у двери и сразу забыл об этом.

Положил is the past tense of положить, a perfective verb. Perfective verbs present an action as a completed whole.

Here that makes sense because the speaker is talking about one finished action:

• he put the keys down,
• and then forgot about it.

Compare:

• положил = put down / placed as a completed act
• клал = was putting / used to put / put repeatedly depending on context

Поставил usually suggests setting something upright or placing something in position, while положил suggests laying something down. With keys on a shelf, положил is the most natural choice.

Положил is past tense, masculine singular.

Russian past tense forms agree with gender and number:

• положил = masculine
• положила = feminine
• положило = neuter
• положили = plural

So this sentence suggests the speaker is male. If the speaker were female, it would be:

Я машинально положила ключи на полку у двери и сразу забыла об этом.

The same applies to забыл.

Ключи is the plural form of ключ (key), and here it is in the accusative plural because it is the direct object of положил.

But for inanimate masculine nouns in the plural, the accusative form is the same as the nominative:

• nominative plural: ключи
• accusative plural: ключи

So although the form looks like nominative, its role in the sentence is accusative.

Because Russian uses different cases depending on whether you mean:

• movement to a place → accusative
• location in a place → prepositional

So:

• на полку = onto the shelf
• на полке = on the shelf

In this sentence, the keys are being moved onto the shelf, so на полку is correct.

Compare:

• Я положил ключи на полку. = I put the keys onto the shelf.
• Ключи лежат на полке. = The keys are on the shelf.

У means by, near, at and it requires the genitive case.

So:

• дверь = door
• у двери = by the door / near the door

Here двери is genitive singular.

The phrase на полку у двери means onto the shelf by the door.

After забыть, Russian can use either a direct object or о / об + prepositional, but the meanings are a little different in feel.

• забыть это = to forget it directly; often a fact, a line, a detail, something specific
• забыть об этом = to forget about it; to stop thinking about the matter/event

In this sentence, забыл об этом is very natural because the speaker forgot about the whole action of putting the keys there.

Also, об is used instead of о before words that begin with certain vowels for easier pronunciation:

• об этом
• but о книге

Машинально means something like:

• automatically
• mechanically
• without thinking
• absentmindedly

It describes the way the speaker acted: he put the keys down almost by reflex, not with deliberate attention.

So it fits very well with the second part: because he did it машинально, he then сразу забыл об этом.

Yes, Russian word order is flexible, and сразу can move around.

In this sentence:

• и сразу забыл об этом = and immediately forgot about it

This is the most neutral and natural placement.

Other possible word orders:

• Я сразу забыл об этом = slightly stronger focus on immediately
• Я машинально положил ключи на полку у двери и забыл об этом сразу = possible, but less natural here

Russian often moves adverbs to shift emphasis, but the original version sounds smooth and idiomatic.

Yes, Russian often omits subject pronouns when the subject is clear from the verb form.

So you could say:

• Машинально положил ключи на полку у двери и сразу забыл об этом.

Because положил and забыл are masculine singular past forms, the subject can be understood from context.

But including Я is also completely normal, especially:

• at the start of a new sentence,
• when there is no previous context,
• or when the speaker wants to be explicit.

So Я here is not strange at all.

Because и joins two verbs with the same subject inside one simple sentence:

• Я положил...
• (я) забыл...

Russian normally does not put a comma before и in this kind of structure.

A comma would be needed if there were separate clauses with their own structure in certain cases, but not here.

Yes. Russian word order is flexible, but changing it changes emphasis more than basic meaning.

Original:

• Я машинально положил ключи на полку у двери и сразу забыл об этом.

Possible variations:

• Я положил ключи машинально на полку у двери и сразу забыл об этом.
• Ключи я машинально положил на полку у двери и сразу забыл об этом.
• На полку у двери я машинально положил ключи и сразу забыл об этом.

These all mean roughly the same thing, but the focus shifts:

• Ключи я... emphasizes the keys
• На полку у двери... emphasizes the location
• машинально near the front emphasizes the absentminded way

The original word order is the most neutral and natural.

Yes, but у двери is more natural here.

Both can mean near the door:

• у двери = by the door, right near it
• около двери = near the door, a bit more general

With household location phrases like a shelf by the door, Russian very often prefers у двери.

So:

• на полку у двери sounds very idiomatic
• на полку около двери is possible, but less compact and less natural in this sentence

Usually полка means shelf, and in this sentence that is the most likely meaning.

So на полку у двери would normally be understood as:

• a shelf by the door,
• perhaps a small hallway shelf where people leave keys.

It is not a grammatical issue, but it is useful vocabulary:

• полка = shelf
• дверь = door
• ключи = keys

The sentence sounds like a very ordinary everyday situation in a Russian home.