Learn Hardcore Russian: Вечером мы идём в театр на новый спектакль. - In the evening we are going to the theater for a new play.

вечер – nominative, “evening” (as a subject: Вечер холодный – The evening is cold).
вечером – instrumental, used for general time expressions:
- Утром – in the morning
- Днём – in the daytime
- Вечером – in the evening
- Ночью – at night

So Вечером мы идём... literally feels like “(During the) evening, we are going…”. Using вечер or вечера here would be ungrammatical in this meaning.

Why is the present tense идём used to talk about the future plan?

In Russian, the present tense of a verb of motion can express a near future arrangement, especially with a time expression:

Вечером мы идём в театр. – We’re going to the theater this evening.
Завтра я лечу в Москву. – I’m flying to Moscow tomorrow.

This is similar to English present continuous (We are going…), but in Russian you just use the simple present. The time word (вечером) clarifies that it’s about the future, not about a habit.

What’s the difference between идём, пойдём and ходим?

All three come from the verb идти/ходить (to go on foot), but differ in aspect and meaning:

идём – present tense of идти (unidirectional, imperfective)
- Вечером мы идём в театр.
  A concrete, planned, one-time movement (and here: a future arrangement).
пойдём – future tense of пойти (perfective)
- Вечером мы пойдём в театр.
  Also a one-time future action, often with a slight nuance of decision / intention / starting to go.
ходим – present tense of ходить (multidirectional, imperfective)
- Мы часто ходим в театр. – We often go to the theater.
  This is used for repeated / habitual actions.

In your sentence, идём fits because it’s a specific plan for this evening.

Why is it в театр, but на новый спектакль?

Russian uses different prepositions for places and events:

в театр – literally “into the theater”.
В
- accusative is used for going into a building / enclosed place:
- в школу (to school)
- в магазин (to the shop)
на спектакль – “to (attend) a performance”.
На
- accusative is used with many events, activities, surfaces:
- на концерт (to a concert)
- на выставку (to an exhibition)
- на лекцию (to a lecture)

So you go в театр (into the building) на спектакль (for the performance).

What case is театр in, and why doesn’t it change its form?

Театр here is in the accusative case after в meaning “into / to”:

идти в + accusative = to go to (a place).

For inanimate masculine nouns ending in a consonant, the accusative form is identical to the nominative:

театр (nom.) → театр (acc.)
магазин (nom.) → магазин (acc.)

So the form doesn’t change, but grammatically it is accusative.

Why is it на новый спектакль and not на новый спектакл? What’s going on with -ль?

Спектакль is a masculine noun ending in -ль. In the accusative (for inanimate masculine nouns), it also stays the same as nominative:

nominative: новый спектакль
accusative: новый спектакль (same form)

The -ль at the end is simply part of the noun’s stem. The full phrase на новый спектакль is:

на
- accusative preposition
новый – masculine accusative singular adjective
спектакль – masculine accusative singular noun

How does новый agree with спектакль?

Russian adjectives must agree with their nouns in:

gender
number
case

Here:

спектакль – masculine, singular, accusative (inanimate)
новый – masculine, singular, accusative

For inanimate masculine, accusative = nominative, so the adjective form is новый (same as dictionary form). That’s why it’s новый спектакль, not новое спектакль or новая спектакль.

Why is the word order Вечером мы идём… and not Мы вечером идём…? Are both possible?

Both are grammatically correct; the difference is in emphasis and style:

Вечером мы идём в театр…
Slight emphasis on when (the evening). Common, neutral.
Мы вечером идём в театр…
Slightly more emphasis on we or on the whole activity as something we’re doing “this evening”.

Russian word order is flexible, and adverbs of time (вечером, завтра, сегодня) are often placed at the beginning of the sentence, but putting them after the subject is also fine in everyday speech.

How would I say “In the evenings, we go to the theater” (habitually), not just this one evening?

To express a repeated / habitual action, use ходим and usually a plural time expression:

Вечерами мы ходим в театр. – In the evenings, we (usually) go to the theater.

Differences:

Вечером мы идём… – this evening, a specific plan.
Вечерами мы ходим… – generally, on evenings (many evenings), as a habit.

Is there any article like “a” or “the” in в театр, на новый спектакль?

Russian has no articles. Words like театр and спектакль have no built-in “a / the” equivalent. Context decides:

Мы идём в театр. → We are going to the theater. / We are going to a theater.
на новый спектакль → to a new performance or to the new performance, depending on what the speakers already know.

In translation, you choose a or the based on English style and context, not on any specific Russian word.

How do you pronounce идём and where is the stress?

Идём is pronounced: [ee-DYOM] (IPA: [ɪˈdʲom])

Stress is on -дём (the second syllable).
The ё always carries stress and is pronounced like “yo” in “your”.
дь is soft because of ё: the д is palatalized.

So: и-ДЁМ (not И-дем).

What’s the plural of спектакль, and would the adjective change?

The plural of спектакль is спектакли.

If you want to say new performances, both the noun and adjective go to plural:

nominative plural: новые спектакли
- В этом театре всегда интересные новые спектакли.

In your sentence, it’s one performance, so it stays новый спектакль.