Я сам позвоню бабушке вечером.

Because позвонить takes the dative case, not the accusative.

• позвонить кому? = to call someone
• So:
  • бабушка = nominative
  • бабушке = dative

This is different from English, where call someone has no visible case marking. In Russian, the person receiving the call is treated like an indirect object:

• Я позвоню бабушке. = I’ll call grandma.
• Он позвонил другу. = He called his friend.

So after звонить / позвонить, you normally use the dative.

Because Russian often uses case endings where English uses prepositions.

In English, you think of it as call to grandma or simply call grandma, but in Russian the idea is built into the noun ending:

• бабушка → бабушке

So the dative ending already shows the role of grandma as the person the call is directed to. No separate to is needed.

Сам adds emphasis: I myself, I’ll do it personally, or I’ll call her on my own.

So:

• Я позвоню бабушке вечером. = I’ll call grandma in the evening.
• Я сам позвоню бабушке вечером. = I’ll call grandma myself this evening.

It often suggests contrast, for example:

• Не надо просить сестру — я сам позвоню бабушке вечером. = No need to ask my sister—I’ll call grandma myself this evening.

So сам is not necessary for the basic meaning, but it adds emphasis.

Not exactly. It can overlap with by myself, but here it mainly means myself / personally.

Compare:

• Я сам это сделаю. = I’ll do it myself.
• Я один это сделаю. = I’ll do it alone.

So in your sentence, сам is more about personal responsibility or emphasis than physical solitude.

Because позвоню is a future form, while звоню is present tense.

• Я звоню бабушке. = I am calling grandma / I call grandma
• Я позвоню бабушке. = I will call grandma

There is also an aspect difference:

• звонить = imperfective
• позвонить = perfective

The perfective verb позвонить is used for a single completed action: to make a call.

So позвоню means I will place the call / I’ll give her a call.

Because in Russian, perfective verbs do not have a true present tense. Their present-looking forms actually refer to the future.

So:

• я позвоню
• ты позвонишь
• он позвонит

all mean future:

• I will call
• you will call
• he will call

This is very common in Russian. With perfective verbs, those forms usually express a future completed action.

Because the sentence is talking about one complete call, not an ongoing or repeated process.

• Я позвоню бабушке вечером.
  = I’ll call grandma this evening.
  A single, complete action.

• Я буду звонить бабушке вечером.
  = I’ll be calling grandma in the evening or I’ll call grandma repeatedly / for some time in the evening, depending on context.

So позвоню is the natural choice when you mean one definite call.

Because Russian often uses the instrumental case in fixed time expressions with parts of the day:

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

So вечером is a standard adverbial form meaning in the evening or this evening.

It is best learned as a common time expression rather than trying to translate the case too literally.

Yes. The exact translation depends on context.

• Я позвоню бабушке вечером. can mean:
  • I’ll call grandma in the evening
  • I’ll call grandma this evening

If you want to be more explicit, you can add a word like:

• сегодня вечером = this evening
• завтра вечером = tomorrow evening

No. Russian word order is flexible, and changing it changes emphasis more than basic meaning.

Your sentence:

• Я сам позвоню бабушке вечером.

is natural and clear. It often sounds like the emphasis is on I myself.

Other possible orders:

• Вечером я сам позвоню бабушке.
  Emphasis on in the evening.

• Бабушке я сам позвоню вечером.
  Emphasis on grandma.

• Я бабушке сам позвоню вечером.
  Also possible, with a slightly different rhythm/emphasis.

So the sentence meaning stays similar, but the focus shifts.

No. A female speaker should say Я сама.

• Я сам позвоню бабушке вечером. = said by a man/boy
• Я сама позвоню бабушке вечером. = said by a woman/girl

That is because сам agrees with the speaker’s gender.

Similarly:

• мы сами = we ourselves

Yes. Then the sentence becomes simply:

• Я позвоню бабушке вечером.

That means I’ll call grandma this evening / in the evening.

Adding сам makes it more emphatic:

• I’ll call her myself.

So сам is optional, but meaningful.

Because Russian often leaves out possessive words like my when the relationship is obvious from context.

So:

• Я позвоню бабушке вечером.

very naturally means I’ll call my grandma this evening.

If needed, you can say:

• Я позвоню своей бабушке вечером.

But in many everyday contexts, that sounds less natural unless you need to stress my own grandma as opposed to someone else’s.

The main word stresses are:

• Я
• сам
• позвоню́
• ба́бушке
• ве́чером

So roughly:

ya sam pazva-NYU BA-boosh-ke VYE-che-ram

A few useful notes:

• позвоню́ has stress on the last syllable
• ба́бушке has stress on the first syllable
• ве́чером has stress on the first syllable

If you say the whole sentence naturally, the strongest sentence stress often falls on сам or позвоню́, depending on what you want to emphasize.