Если не утром, то вечером мы всё равно зайдём в магазин.

This is a very common Russian pattern:

если не X, то Y = if not X, then Y / if not X, at least Y

So here:

• если не утром = if not in the morning
• то вечером = then in the evening

It does not necessarily sound like a strict logical condition. In everyday speech, it often means something like:

• maybe not X, but then Y
• if X doesn’t work, Y will

So the sentence suggests: We’ll go to the store anyway — if not in the morning, then in the evening.

Both утром and вечером are forms of the instrumental case, but in these expressions they function like adverbs of time:

• утро → утром = in the morning
• вечер → вечером = in the evening

This is a standard Russian way to talk about parts of the day.

Compare:

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

So although they come from nouns, here they are best learned as fixed time expressions.

Because Russian normally does not use a preposition with these common time expressions.

You say:

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

So в утром and в вечером are incorrect.

This is just one of those structures you should memorize as a set phrase. English uses in the morning, but Russian uses just the noun form by itself.

Всё равно means anyway, all the same, or in any case.

In this sentence, it emphasizes that the action will happen no matter which time works:

• мы всё равно зайдём в магазин = we’ll stop by the store anyway

So even if the morning doesn’t happen, the evening will.

Very often всё равно expresses:

• lack of difference: It makes no difference
• persistence despite circumstances: anyway

Here it is the second meaning.

Examples:

• Я всё равно пойду. = I’ll go anyway.
• Он всё равно не согласится. = He still won’t agree anyway.

Зайдём is the 1st person plural future of зайти, a perfective verb.

Here зайти в магазин means:

• to stop by the store
• to drop into the store
• sometimes simply to go into the store

So the nuance is often a short visit or stopping in on the way.

That is slightly different from:

• пойдём в магазин = we’ll go to the store
• сходим в магазин = we’ll go to the store and come back
• зайдём в магазин = we’ll stop by / pop into the store

In many contexts, English may translate all of them simply as go to the store, but the Russian verb adds nuance.

Because the sentence refers to a single completed future action.

Russian often uses a perfective future for this:

• зайти = perfective
• зайдём = we will stop by

Why not the others?

• идём = we are going / we go
  This is present tense, so it does not fit well here.
• заходим = we go in / we are dropping by
  Also present, or habitual.
• пойдём = we will go
  Possible in some contexts, but it means set off/go, not specifically stop by.
• зайдём highlights one completed future visit.

So зайдём is used because the speaker means a definite future visit to the store.

Because with verbs of motion meaning go into a place, Russian uses в + accusative.

Here:

• магазин is the noun
• в магазин = into/to the store

This is the normal pattern for entering or going to places conceived as enclosed spaces or destinations.

Examples:

• в школу = to school
• в банк = to the bank
• в ресторан = to the restaurant

You would use к with people or places seen more as points/appointments:

• к врачу = to the doctor
• к другу = to a friend’s place

So зайти в магазин is the standard expression.

The commas mark the structure Если не X, то Y.

So Russian punctuation reflects the two-part construction:

• Если не утром, то вечером

The first comma separates the first part from the second. The second comma separates that opening phrase from the main clause:

• Если не утром, то вечером, мы всё равно зайдём в магазин.

In modern writing, many people write exactly the learner sentence:

Если не утром, то вечером мы всё равно зайдём в магазин.

The key point for a learner is that если не ..., то ... is treated as a paired structure, and the comma before то is expected.

Yes, very often.

Russian verb endings already show the subject:

• зайдём clearly means we will stop by

So you could say:

• Если не утром, то вечером всё равно зайдём в магазин.

That would still be natural.

Including мы can:

• add emphasis
• make the sentence clearer
• slightly highlight we, especially if contrasting with someone else

So both versions are possible, but Russian often omits subject pronouns when they are obvious.

Russian word order is flexible, and this sentence is arranged to foreground the time alternatives first:

• Если не утром, то вечером = first we hear the choice of time
• мы всё равно зайдём в магазин = then we hear the main message

This order sounds natural because the speaker is setting up the scheduling contrast before stating the action.

The sentence could be rearranged, but the emphasis would change. For example:

• Мы всё равно зайдём в магазин, если не утром, то вечером.

This version puts the main fact first: we’ll go anyway, and then adds the timing.

So the original order emphasizes when, while the alternative order emphasizes the fact that it will happen.

Yes.

In this sentence, утром and вечером are neutral, standard time expressions:

• утром = in the morning
• вечером = in the evening

But similar expressions can have slightly different meanings:

• с утра = since morning / from the morning onward
• под вечер = toward evening / by evening

So:

• если не утром, то вечером is the simplest and most neutral way to contrast two times of day.
• Using с утра or под вечер would change the nuance and would not be a direct replacement here.

For example:

• С утра я занят. = I’m busy from the morning on / since morning
• Под вечер стало холодно. = Toward evening it got cold

So the original sentence uses the most straightforward forms for in the morning / in the evening.