Learn Hardcore German: Heute planen wir einen Serienabend im Wohnzimmer. - Today we are planning a series night in the living room.

Heute planen wir …
- 1st position: Heute (one element: “today”)
- 2nd position: planen (the finite verb)
- 3rd position: wir (the subject)

In German, the subject doesn’t have to be first; what matters is that the verb stays in second position.
So:

Heute planen wir einen Serienabend im Wohnzimmer.
Wir planen heute einen Serienabend im Wohnzimmer.

Both are correct, as long as the verb is in second position.

Can I also say Wir planen heute einen Serienabend im Wohnzimmer? Is there a difference in meaning?

Yes, that sentence is completely correct:

Heute planen wir einen Serienabend im Wohnzimmer.
Wir planen heute einen Serienabend im Wohnzimmer.

Both mean: “Today we’re planning a series night in the living room.”

The difference is emphasis:

Heute planen wir … puts a bit more focus on today (contrast with other days).
Wir planen heute … is a bit more neutral, more like normal spoken order.

In everyday speech, Wir planen heute … may sound slightly more natural, but both are fine.

Why is it einen Serienabend and not ein Serienabend?

Because Serienabend is masculine (der Serienabend) and it is the direct object of the sentence, so it must be in the accusative case.

Nominative (subject): ein Serienabend
Accusative (direct object): einen Serienabend

Pattern for the indefinite article ein- in the singular:

Masculine: ein → einen (Akkusativ)
Feminine: eine → eine
Neuter: ein → ein

Since we are planning something (what? a series night), einen Serienabend is correct.

How do I know that Serienabend is masculine?

The gender usually comes from the last part of a compound noun.

Serienabend = Serien (plural of die Serie) + Abend (der Abend)

Abend is masculine: der Abend.
In compounds, the head noun is usually the last part, so Abend determines the gender.

Therefore:

der Serienabend
einen Serienabend (accusative masculine)

Why is Serienabend written as one word and capitalized?

In German, nouns are always capitalized, and compound nouns are written as one word.

Serie (series, show) + Abend (evening) → Serienabend (literally “series-evening”)

So you don’t write Serien Abend or serien abend, but:

ein Serienabend
einen Serienabend

This is very typical for German: Fernsehabend, Spieleabend, Filmabend, etc.

What exactly does Serienabend mean? Is it just “evening with series”?

Yes, literally it is “series evening”, but in natural English it’s more like:

“a TV show night”
“a series marathon evening”
“an evening where we watch several episodes of a series”

It usually implies:

watching one or more series,
typically several episodes,
often planned as a cozy event (snacks, drinks, etc.).

So einen Serienabend planen = planning an evening dedicated to watching series (on TV or streaming).

Why is it im Wohnzimmer and not in dem Wohnzimmer?

im is simply the contracted form of in dem.

in
- dem → im

So:

in dem Wohnzimmer (in the living room)
im Wohnzimmer (exactly the same meaning, just shorter and more natural)

This contraction is very common:

in dem → im
an dem → am
zu dem → zum
bei dem → beim

Why is Wohnzimmer in the dative case after in? I thought in can take the accusative too.

The preposition in can take dative or accusative depending on the meaning:

Dative = location (where?) → position
Accusative = direction (where to?) → movement

In your sentence:

im Wohnzimmer = in dem Wohnzimmer = in the living room (location → dative)

Examples:

Wir sind im Wohnzimmer. (We are in the living room.) → dative
Wir gehen ins Wohnzimmer. (We are going into the living room.)
- in das Wohnzimmer → ins → accusative (movement)

So here it’s im Wohnzimmer because it describes where the series night will take place, not movement.

Is Wohnzimmer neuter? How do I see that in the sentence?

Yes, Wohnzimmer is neuter:

das Wohnzimmer (the living room)

In the sentence you see this from the article form in dative:

Dative masculine: dem (e.g. in dem Keller)
Dative neuter: dem (same form)
Contracted with in: im

So im Wohnzimmer = in dem Wohnzimmer = dative singular.
You don’t see the gender directly from im, but you know from the dictionary that:

das Wohnzimmer = neuter

Can German present tense like planen refer to the future, like “we are going to plan” or “we will plan”?

Yes. German very often uses the present tense to talk about the near future, especially when the time is clear from context or a time word like heute (“today”) is used.

Heute planen wir einen Serienabend …
= Today we’re planning a series night …
(future meaning, but present tense form)

You could also say:

Heute werden wir einen Serienabend im Wohnzimmer planen.

This is grammatically correct but sounds more formal or heavier. The simple present planen is more natural in everyday speech here.

Could I also say Heute machen wir einen Serienabend im Wohnzimmer instead of planen?

Yes, and in many real-life contexts machen would actually be more common:

Heute machen wir einen Serienabend im Wohnzimmer. = Today we’re having a series night in the living room.

Subtle difference:

planen = focusing on the act of planning/organizing.
machen = focusing on actually doing/having the series night.

If the meaning shown to the learner was “Today we’re having a series night…”, machen fits a bit better.
If it is really “Today we are planning (organizing) a series night…”, then planen is accurate.

Is it important that Heute comes first, then the verb, then the subject, or could I say Heute wir planen einen Serienabend?

You cannot say Heute wir planen …. That breaks the verb-second rule.

Correct patterns:

Heute planen wir einen Serienabend im Wohnzimmer.
Wir planen heute einen Serienabend im Wohnzimmer.

In a main clause:

You can move different elements (time, place, object) to the first position for emphasis.
But the finite verb must always be in second position.
The subject can come after the verb.

So Heute planen wir … is fine, but Heute wir planen … is incorrect.