Я должен был позвонить бабушке вечером, но совсем забыл.

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Breakdown of Я должен был позвонить бабушке вечером, но совсем забыл.

• но: but
• бабушка: the grandmother
• вечером: in the evening
• забыть: to forget
• позвонить: to call
• совсем: completely
• должен был / должна была / должны были: should (past)

Questions & Answers about Я должен был позвонить бабушке вечером, но совсем забыл.

What exactly does должен был mean here, and why are there two words?

Должен by itself means “must, have to, ought to” (obligation, duty) in the present or general sense.

To talk about obligation in the past, Russian adds the past tense of быть (to be):

• должен – “must / have to” (now / generally)
• должен был – “had to / was supposed to (but in the past)”

So Я должен был позвонить… literally is “I had to call / I was supposed to call…”.

Compare:

• Я должен позвонить бабушке. – I have to call Grandma. (now / in the near future)
• Я должен был позвонить бабушке. – I was supposed to call Grandma. (in the past)

Why is it должен был позвонить (infinitive) and not something like должен был позвонил?

After должен (был/была/...), Russian always uses the infinitive to express what you “have to” do:

• должен был + infinitive

So:

• Я должен был позвонить. – I was supposed to call.
• Он должен был уйти. – He was supposed to leave.
• Мы должны были прийти раньше. – We were supposed to come earlier.

You never combine должен был with a past-tense verb (должен был позвонил is wrong). The obligation is in the past (должен был), and the action is expressed by the infinitive (позвонить).

Why is it бабушке and not бабушку after позвонить?

Because звонить / позвонить take the dative case for the person you call:

• звонить / позвонить кому? – to call to whom?

Бабушке is the dative form of бабушка:

• бабушка (nom.) → бабушке (dat.)

So:

• позвонить бабушке – to call (one’s) grandmother.
• позвонить другу – to call a friend.
• позвонить маме – to call Mom.

Using бабушку (accusative) here would be ungrammatical with позвонить.

Why is it позвонить and not звонить? What’s the difference?

Звонить and позвонить are imperfective / perfective partners:

• звонить (impf.) – to be calling, to call in general, repeated / ongoing action.
• позвонить (pf.) – to make a (single, completed) call.

In Я должен был позвонить бабушке…, the speaker had an obligation to make one specific call, seen as a complete act. That fits the perfective позвонить.

Compare:

• Я часто звонил бабушке. – I used to call Grandma often. (repeated, impf.)
• Я должен был позвонить бабушке. – I was supposed to make a call to Grandma. (one specific action, pf.)

Why is вечером used without any preposition? Why not в вечером or something similar?

In Russian, many simple time expressions are used without a preposition and take the instrumental to mean “in/at (time)”:

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

So:

• Я должен был позвонить бабушке вечером. – I was supposed to call Grandma in the evening.

If you say в вечер, it’s usually part of a more complex phrase (e.g. в этот вечер – on that evening), but “in the evening” by itself is simply вечером.

What nuance does совсем add in но совсем забыл?

Совсем here means completely / totally / entirely.

So:

• Я забыл. – I forgot.
• Я совсем забыл. – I completely forgot / it slipped my mind entirely.

Other uses:

• Он совсем не устал. – He is not tired at all. (совсем + не = “not at all”)
• Она совсем другая. – She is completely different.

In your sentence, совсем intensifies the forgetting: not just “I forgot”, but “I totally forgot.”

Why is there no second я before забыл? Why not …, но я совсем забыл?

Russian often omits repeated subjects when it’s obvious they’re the same as before.

• Я должен был позвонить бабушке вечером, но совсем забыл.

The subject я is clear from context, so it doesn’t have to be repeated.

Both versions are correct:

• … но совсем забыл. – more natural, lighter.
• … но я совсем забыл. – also correct; adding я slightly emphasizes “but I completely forgot.”

Why is забыл masculine? What if the speaker is female?

Russian past tense verbs agree in gender and number with the subject.

Here забыл is:

• masculine, singular, past: “(he / a man) forgot”.

If the speaker (я) is female:

• Я совсем забыла. – I completely forgot. (female speaker)

Forms:

• забыл – he / a man / masc. “forgot”
• забыла – she / a woman / fem. “forgot”
• забыло – neuter subject “forgot”
• забыли – they / we / you(pl) “forgot”

The same applies to должен был:

• я должен был – if the speaker (я) is male
• я должна была – if the speaker is female

How would the sentence change if the speaker is a woman?

Both verbs in the past would change to feminine:

• Я должна была позвонить бабушке вечером, но совсем забыла.

Changes:

• должен был → должна была
• забыл → забыла

What’s the difference in English between “I had to call” and “I was supposed to call” here? Which is closer to должен был?

Должен был is flexible; it can mean either:

• external duty/necessity – “I had to call Grandma (it was necessary, a duty).”
• expected / planned action – “I was supposed to call Grandma.”

In this context (followed by но совсем забыл), “I was supposed to call Grandma in the evening, but I completely forgot” usually sounds more natural in English, because it implies an expected action that didn’t happen.

But “I had to call Grandma in the evening, but I completely forgot” is also an acceptable translation.

Can I move вечером to another position, like Я вечером должен был позвонить бабушке…? Does the meaning change?

Yes, you can move вечером. Russian word order is flexible and mainly affects emphasis, not basic meaning.

All of these are grammatical:

1. Я должен был позвонить бабушке вечером, но совсем забыл.
   – Neutral: focus on the obligation (“I was supposed to call Grandma in the evening…”).

2. Я вечером должен был позвонить бабушке, но совсем забыл.
   – Slightly more emphasis on вечером (“It was in the evening that I was supposed to call Grandma…”).

3. Вечером я должен был позвонить бабушке, но совсем забыл.
   – Strongest focus on the time (“In the evening, I was supposed to call Grandma, but I completely forgot.”).

Meaning stays the same; only what you’re highlighting changes.

What’s the difference between надо было позвонить and должен был позвонить?

Both express that a call should have been made in the past, but the nuance is a bit different:

• надо было позвонить – “(one) needed to call / it was necessary to call”
  More impersonal, focuses on necessity or good idea:

  • Мне надо было позвонить бабушке. – I should have called Grandma / I needed to call Grandma.

• должен был позвонить – “was supposed to / had to call”
  Sounds more like a duty or explicit obligation:

  • Я должен был позвонить бабушке. – I was supposed to call Grandma (it was my responsibility).

Often they can both translate as “should have called”, but должен был is a bit stronger on obligation.