Learn Hardcore German: Wir lassen das Glas nicht fallen.

Wir lassen das Auto reparieren. – We’re having the car repaired. (We let/cause the car be repaired.)
Ich lasse ihn sprechen. – I let him speak.

Why can’t I say Wir fallen das Glas nicht?

Because fallen in German is intransitive: something falls by itself. It cannot directly take an object.

Correct: Das Glas fällt. – The glass falls.
Incorrect: Wir fallen das Glas. – This tries to make “fall” into a transitive verb, which German does not allow.

To express that we cause the glass to fall, German needs lassen:

Wir lassen das Glas fallen. – We drop the glass.
With negation:
Wir lassen das Glas nicht fallen. – We don’t drop the glass.

Why is fallen at the very end of the sentence?

In a normal main clause, German word order is:

Position 1: (often the subject) – Wir
Position 2: the conjugated verb – lassen
Middle field: other elements – das Glas nicht
End field: infinitives / participles – fallen

So:

Wir | lassen | das Glas nicht | fallen.

Any extra infinitive (here: fallen) is pushed to the end of the clause.
This also happens with modal verbs and perfect tense:

Wir wollen das Glas nicht fallen lassen.
Wir haben das Glas nicht fallen lassen.

Why is nicht before fallen? Could it go somewhere else?

Default rule in sentences like this:
nicht comes before the final infinitive (or verb cluster) that it negates.

Wir lassen das Glas nicht fallen.
= We do not drop the glass.

You can move nicht for emphasis or to negate different parts, but the basic, neutral version is:

Wir lassen das Glas nicht fallen. – We don’t drop the glass. (neutral)

Compare:

Wir lassen nicht das Glas fallen, sondern den Teller.
= We are not dropping the glass, but the plate.
Here nicht focuses on das Glas, contrasting it with something else.

For a learner, the safest, most natural word order here is exactly the given sentence:
Wir lassen das Glas nicht fallen.

What case is das Glas in, and why is the article das?

das Glas is in the accusative case as the direct object of lassen.

Subject (nominative): Wir – we
Verb: lassen – we let/cause
Direct object (accusative): das Glas – the thing affected by the action

The noun Glas is neuter in German, so its definite article is das in both nominative and accusative:

Nominative: das Glas
Accusative: das Glas

So the form das doesn’t change here, but the grammatical function is accusative.

What tense is this, and can it really mean “we won’t drop the glass” (future)?

Grammatically, this is the present tense:

lassen conjugated for wir → wir lassen

But German very often uses the present tense with a future meaning, especially when the context is clearly about the future:

Wir lassen das Glas nicht fallen.
= We don’t drop the glass. / We’re not going to drop the glass.

If you want to make the future meaning very explicit, you can use werden + infinitive:

Wir werden das Glas nicht fallen lassen.
= We will not drop the glass.

Both are correct; the simple present is more common in everyday speech when the time is clear from context.

How would I say “We didn’t drop the glass” in the past tense?

The most natural form in spoken German is the Perfekt (present perfect):

Wir haben das Glas nicht fallen lassen.
Literally: “We have not let the glass fall.”

Grammar breakdown:

haben – auxiliary verb (present tense, for wir)
fallen lassen – past participle / infinitive cluster at the end

You would not say:

✗ Wir ließen das Glas nicht fallen.

This is grammatically correct (Präteritum of lassen), but in everyday spoken German it sounds quite formal or literary. It can appear in written narrative, though.

What is the difference between Wir lassen das Glas nicht fallen and Wir lassen nicht das Glas fallen?

Both are grammatically possible, but they emphasize different things:

Wir lassen das Glas nicht fallen.
– Neutral negation: we are not dropping the glass (at all).
– This is what you’d normally say.
Wir lassen nicht das Glas fallen, sondern den Teller.
– Contrastive focus: it’s not the glass that we are dropping, but something else (here: the plate).
– Without a contrasting part (sondern …), it sounds odd and incomplete.

So, unless you are contrasting glass with some other object, use the first sentence.

Does lassen here mean “to leave (behind)” like in etwas liegen lassen?

Here, lassen is used in its causative / “let” sense:
lassen + infinitive = make/allow something do something.

Wir lassen das Glas (nicht) fallen.
= We (do not) let the glass fall. → We (don’t) drop the glass.

lassen can indeed have other meanings in different contexts:

To leave something somewhere
- Ich lasse mein Handy zu Hause. – I’m leaving my phone at home.
- Lass das Glas hier stehen. – Leave the glass here (standing).
To stop doing something / give up
- Ich lasse das Rauchen. – I’m quitting smoking.
- Lass das! – Stop that!
Causative “have something done”
- Ich lasse mein Auto waschen. – I’m having my car washed.

In your sentence, it’s sense 3 (causative), just that the action is fallen instead of “wash/repair/…”.

How would I say “Don’t drop the glass!” as a command?

Use the imperative of lassen with nicht and fallen:

Informal du:
Lass das Glas nicht fallen! – Don’t drop the glass!
Informal ihr (speaking to more than one person you know):
Lasst das Glas nicht fallen!
Formal Sie:
Lassen Sie das Glas nicht fallen!

Notice:

The conjugated form of lassen comes first (command).
das Glas is the object in the middle.
nicht fallen (the negation + infinitive) stay at the end.