Иногда вечером я забываю отключать телефон.

In Russian, parts of the day are usually used in the instrumental case without a preposition to mean “in the …” (time of day):

• утром – in the morning
• днём – in / during the day
• вечером – in the evening
• ночью – at night

So вечером is the instrumental singular of вечер, and it functions adverbially: “in the evening”.

В вечер is normally not used for time of day in modern Russian. You might see в этот вечер (on this evening) or в тот вечер (that evening), but that’s with a demonstrative word, not bare вечер.

They answer different questions:

• иногда – how often? → sometimes (frequency)
• вечером – when? → in the evening (time of day)

So:

• Иногда вечером я забываю отключать телефон.
  → Sometimes in the evening I forget to turn off my phone.

If you say only:

• Вечером я забываю отключать телефон.
  it sounds more like “In the evening I (usually / often) forget to turn off my phone” – it suggests that this typically happens in the evening, not just occasionally.

If you say only:

• Иногда я забываю отключать телефон.
  then it’s “Sometimes I forget to turn off my phone” with no information about the time of day.

So both words are useful and not redundant: иногда talks about frequency, вечером about time.

The word order is not rigid here, and several versions are natural:

• Иногда вечером я забываю отключать телефон.
  (Your original sentence – very natural, slight emphasis on sometimes in the evening.)

• Я иногда вечером забываю отключать телефон.
  Also natural. Now the sentence starts with я, but the meaning is practically the same.

• Иногда я вечером забываю отключать телефон.
  Also possible; you’re splitting иногда and вечером, but both still relate to the verb.

Less natural:

• Я забываю иногда вечером отключать телефон.
  Grammatically possible, but sounds clumsy and marked; in everyday speech Russians usually put иногда / вечером before the verb, not between verb and infinitive.

General guideline: neutral Russian tends to keep adverbs of time/frequency near the beginning of the clause and before the verb. Changing the order usually changes emphasis more than meaning.

Я забываю is present tense, imperfective aspect of забывать.

In this context with иногда, it describes a repeated / habitual action. Depending on context, it can correspond to several English forms:

• I sometimes forget
• I keep forgetting
• I usually end up forgetting (in some contexts)

Russian doesn’t have a special present continuous form like “I am forgetting”; я забываю can mean either I forget (generally) or I am forgetting (right now), depending on context. Here, with иногда вечером, it clearly means a repeated habit, not something happening just this moment.

The difference is in aspect of the infinitive:

• отключать – imperfective (focus on process / repeated action)
• отключить – perfective (focus on a single, complete act)

With забывать / забыть, both aspects are possible, but they feel a bit different:

1. Иногда вечером я забываю отключать телефон.

   • Imperfective infinitive.
   • Emphasizes that the whole situation “turning the phone off (as a usual action)” is what you tend to forget.
   • Feels more like a general habit: Sometimes, in the evenings, I (tend to) forget about the practice of turning my phone off.

2. Иногда вечером я забываю отключить телефон.

   • Perfective infinitive.
   • Feels a bit more like repeated individual events: on some evenings, you forget to perform that one act of turning it off.
   • Still habitual in meaning because of иногда, but the focus is on each separate “turn-off” event.

In everyday speech, both variants are possible and close in meaning. In your sentence, the imperfective отключать fits very naturally with the idea of a recurring habit.

In Russian, when you say “forget to do something”, you use:

забывать / забыть + infinitive

So:

• я забываю отключать телефон – I forget to turn off my phone
• я забыл позвонить – I forgot to call
• он часто забывает есть – he often forgets to eat

Using отключаю (a finite verb) after забываю would be ungrammatical:

• ✗ я забываю отключаю телефон – incorrect

The infinitive отключать here is the thing you are forgetting to do.

In Russian, the possessive мой / твой / его / её, etc. is often omitted when it’s obvious from context whose thing it is.

Commonly omitted with:

• body parts: я мою руки (I wash my hands, literally I wash hands)
• close family: я звоню маме (I’m calling my mom)
• personal items: я потерял телефон (I lost my phone)

So in:

• Иногда вечером я забываю отключать телефон.

it is naturally understood as “my phone” without needing мой. You can say мой телефон for emphasis or contrast:

• Иногда вечером я забываю отключать мой телефон, а не рабочий.
  Sometimes in the evening I forget to turn off *my phone, not the work one.*

Телефон is in the accusative case as the direct object of отключать:

• отключать что? – телефон

For inanimate masculine nouns like телефон, the accusative singular form is identical to the nominative:

• Nominative: телефон лежит на столе. – The phone is on the table.
• Accusative: я отключаю телефон. – I turn off the phone.

So even though the function changed (subject → object), the form stays the same.

Both can be used with телефон, but there are nuances:

• выключать телефон

  • Basic, everyday verb for “to turn a device off (cut the power)”.
  • Opposite of включать телефон – to turn the phone on.

• отключать телефон

  • Literally “to disconnect the phone”.
  • Can mean turning it off, disabling its network connection, putting it into airplane mode, or generally making it not function / not reachable.
  • Also used in contexts like отключить интернет, отключить воду – disconnect the internet / water.

In casual speech about your own mobile, выключать телефон is very common, but отключать телефон is also widely understood and used. In your sentence, отключать is perfectly natural.

Yes, you can, and both are correct, but they feel slightly different:

• Иногда вечером я забываю отключать телефон.
  – Sometimes in the evening I forget to turn off my phone.
  Neutral: some evenings, at some point in the evening, you forget.

• Иногда по вечерам я забываю отключать телефон.
  – Literally Sometimes in the evenings I forget to turn off my phone.
  По вечерам / вечерами has a stronger “on evenings as a regular time period” feel – a more habitual, repeated timeframe.

The difference is subtle; both can be used to talk about a recurring problem that happens on some evenings.

No. Я забываю телефон means something different:

• Я забываю телефон.
  → I (tend to) forget my phone (somewhere / at home, etc.).

Here телефон is a thing you forget/leave behind.

To say “I forget to turn off my phone”, you need the infinitive construction:

• я забываю отключать телефон
  → I forget to turn off my phone.
• я забываю выключить телефон
  → I forget to switch off my phone.

So:

• забывать + noun → forgetting the thing itself
• забывать + infinitive → forgetting to do an action

Yes. Russian often drops the subject pronoun when it’s clear from the verb ending:

• Иногда вечером я забываю отключать телефон.
• Иногда вечером забываю отключать телефон.

Both are correct. The version without я is a bit more informal / conversational and slightly more impersonal in tone, but the meaning is the same: Sometimes in the evening I forget to turn off my phone.