Learn Hardcore German: Wir müssen heute eine Entscheidung treffen.

In your sentence, wir goes with müssen, so you get wir müssen.
The umlaut drops in some forms (muss, musst, müsst), but the infinitive and wir/sie/Sie forms keep it: müssen.

Why do we have müssen plus another verb treffen at the end?

In German, modal verbs (like müssen, können, wollen, dürfen, sollen, mögen) are usually followed by another verb in the infinitive.

The structure is:

Subject + conjugated modal verb + ... + infinitive at the end

So:

Wir – subject
müssen – conjugated modal verb (1st person plural)
heute – time adverb
eine Entscheidung – object (what?)
treffen – infinitive at the end

Literally: We must today a decision make.
Correct idiomatic English: We have to make a decision today.

Putting treffen at the end is normal German word order in main clauses with a modal verb.

Why is heute in the middle of the sentence? Can it move?

In main clauses, German has the verb-second (V2) rule: the conjugated verb must be in second position. Other elements can be relatively flexible.

Your sentence:

Wir (1) müssen (2) heute eine Entscheidung treffen.

Here müssen is second; heute sits comfortably after the verb.

You can also say:

Heute müssen wir eine Entscheidung treffen.
(Now Heute is first, müssen stays second, wir moves after the verb.)

Both sentences are correct. The position of heute changes the emphasis slightly:

Wir müssen heute… – focus on we
Heute müssen wir… – focus on today (today, not another day)

Why is it eine Entscheidung treffen and not eine Entscheidung machen?

This is a matter of collocation (which words naturally go together).

In German you usually:

eine Entscheidung treffen – literally “to hit/meet a decision”, but idiomatically “to make a decision”.

Using machen here (eine Entscheidung machen) sounds wrong or at best very non‑native.

Other similar fixed combinations with treffen:

eine Wahl treffen – to make a choice
eine Auswahl treffen – to make a selection

So you should learn eine Entscheidung treffen as a fixed expression, just like English has “make a decision”, not “do a decision”.

Why is it eine Entscheidung, not ein Entscheidung?

Because Entscheidung is:

feminine: die Entscheidung
and here it is the direct object (accusative case).

The indefinite article eine is used for feminine singular in both nominative and accusative cases:

Nominative: Eine Entscheidung ist schwierig. – A decision is difficult.
Accusative: Wir treffen eine Entscheidung. – We (make) a decision.

With grammatically masculine or neuter nouns, the form would be ein:

ein Plan (masc.)
ein Problem (neut.)

But Entscheidung is feminine, so you need eine.

How do I know that Entscheidung is feminine?

You usually have to learn the gender with each noun, but there are helpful patterns.

One strong pattern: nouns ending in -ung are almost always feminine:

die Entscheidung – the decision
die Wohnung – the apartment
die Zeitung – the newspaper
die Bedeutung – the meaning

So when you see -ung, you can very confidently assume it’s die and that the correct indefinite article is eine.

Does treffen here still mean “to meet”?

Literally, treffen means to hit / to meet (a target or a person):

Ich treffe meinen Freund. – I meet my friend.
Die Kugel trifft das Ziel. – The bullet hits the target.

But treffen is also used in several fixed expressions where it takes on a more abstract sense, e.g.:

eine Entscheidung treffen – to make a decision
Vorbereitungen treffen – to make preparations
Maßnahmen treffen – to take measures

In your sentence, it doesn’t mean “to meet a decision” in a literal sense; it’s just the standard idiomatic verb that combines with Entscheidung to produce “to make a decision”.

Can I say Wir müssen heute entscheiden instead? Is that the same?

Yes, you can say:

Wir müssen heute entscheiden.

That means: We have to decide today.

The difference is:

Wir müssen heute eine Entscheidung treffen.
– Uses the noun Entscheidung
- treffen, a very common phrase.
Wir müssen heute entscheiden.
– Uses the verb entscheiden directly, no object.

In everyday speech they’re very close in meaning, and both are completely natural. The version with Entscheidung sometimes sounds a bit more concrete or slightly more formal, but not by much.

Can I say Wir müssen eine Entscheidung heute treffen or Wir müssen eine Entscheidung treffen heute?

Both are grammatically possible, but they sound less natural than the original:

Most natural standard order:
Wir müssen heute eine Entscheidung treffen.

Putting heute later tends to feel marked (unusual) in standard German:

Wir müssen eine Entscheidung heute treffen. – OK but slightly off; sounds like you’re stressing today in contrast to another time.
Wir müssen eine Entscheidung treffen heute. – Also possible, but quite emphatic/colloquial, often used in spoken language for dramatic stress.

As a safe rule: put simple time adverbs like heute, morgen, jetzt directly after the verb or at the very beginning of the sentence.

What exactly does müssen express here? Is it like “must” or “should”?

In this sentence, müssen expresses a strong necessity or obligation, close to English “must / have to”:

Wir müssen heute eine Entscheidung treffen.
We have to / must make a decision today.

Comparison:

müssen – strong necessity or duty
- Ich muss arbeiten. – I have to work.
sollen – expectation, recommendation, moral duty, or “should”
- Ich soll arbeiten. – I’m supposed to work / I should work.

So Wir müssen heute… means there is no real choice; the decision has to happen today.

Why is müssen in second position? Could wir be somewhere else?

German main clauses follow the verb-second (V2) rule:

The conjugated verb is always in second position.
Only one element can be in front of it (subject, adverb, object, etc.).

In your version:

Wir – first element
müssen – conjugated verb (must be in 2nd place)
The rest – heute eine Entscheidung treffen

If you change the first element, the verb stays second:

Heute müssen wir eine Entscheidung treffen.
Eine Entscheidung müssen wir heute treffen.

So wir can move, but müssen must stay in the second slot in a main clause.

How would I say “We don’t have to make a decision today” vs. “We must not make a decision today”?

German distinguishes these two ideas clearly.

We don’t have to make a decision today
(It’s not necessary.)
Wir müssen heute keine Entscheidung treffen.

You negate the noun phrase (keine Entscheidung), not the verb, to say “no decision is necessary”.
We must not make a decision today
(It’s forbidden.)
Wir dürfen heute keine Entscheidung treffen.

Or more literally with müssen:
Wir dürfen heute keine Entscheidung treffen. is the natural way.
Wir müssen heute keine Entscheidung treffen means “we don’t have to…”, not “we must not…”.

So:

müssen + keine → no necessity
dürfen + keine → not allowed / must not

If I change the object, does the structure stay the same?

Yes, the overall pattern stays the same:

Wir müssen heute [object in accusative] [infinitive]

Examples:

Wir müssen heute den Vertrag unterschreiben.
– We have to sign the contract today.
(masculine: den Vertrag)
Wir müssen heute ein Problem lösen.
– We have to solve a problem today.
(neuter: ein Problem)
Wir müssen heute die Präsentation vorbereiten.
– We have to prepare the presentation today.
(feminine: die Präsentation)

Only the article and noun ending change with gender and number; the overall word order and the modal‑verb structure remain the same.