Learn Hardcore German: Sie ließ das Licht absichtlich an, damit wir den Weg sehen. - She deliberately left the light on so that we can see the way.

ließ is the simple past (Präteritum) of lassen. Here it means “left/kept” (she left the light on). In spoken German, many speakers prefer the present perfect:

Sie hat das Licht absichtlich an gelassen.
Also common: Sie hat das Licht absichtlich angelassen.

How is lassen being used here?

It’s the “leave something in a state” use: lassen + object + predicate (adjective/adverb).

Sie ließ das Licht an. = She left the light on. Comparable patterns:
Ich ließ die Tür offen.
Wir lassen das Fenster zu.

Why is an at the end of the clause? Is it a separable prefix?

In this sentence, an functions as a predicative adverb (“on”), describing the state of the object (das Licht). In main clauses, the finite verb (ließ) sits in position 2, and non-finite or particle-like elements often appear at the end:

Sie ließ das Licht … an. Note: There is also a separable verb anlassen (e.g., “start an engine” or “leave on”), which yields the participle angelassen. With lights, both analyses are used in practice (see next Q/A).

Which is correct in the perfect: an gelassen or angelassen?

For lights, both are used and accepted:

Pattern “lassen + an” → Sie hat das Licht an gelassen.
Separable verb anlassen → Sie hat das Licht angelassen. However, with meanings like “start the engine,” only the verb anlassen works:
Er hat den Motor angelassen. (not: an gelassen)

Why use damit and not um … zu?

damit + finite clause is used when the subject of the purpose clause can be different from the main clause.
um … zu + infinitive requires the subject to be the same as in the main clause. Here the subjects differ (sie vs. wir), so damit is required:
Correct: … damit wir den Weg sehen.
If the subject were the same: Sie ließ das Licht an, um den Weg zu sehen.

Why is sehen in the present tense when the main clause is in the past?

Purpose clauses with damit often use present tense to express the intended result relative to the time of the main action. You can also say:

… damit wir den Weg sehen können. (focuses on ability)
… damit wir den Weg sehen konnten. (also possible; makes the “ability” explicitly past) But … damit wir den Weg sahen is unusual and generally avoided.

Should I add können after sehen?

Both are fine:

… damit wir den Weg sehen. (short, idiomatic)
… damit wir den Weg sehen können. (explicit emphasis on ability) In everyday speech, many speakers prefer the version with können here.

Why is the verb at the end in damit wir den Weg sehen?

damit introduces a subordinate clause, and subordinate clauses in German send the conjugated verb to the end:

… damit [wir den Weg sehen]. Main clauses use verb-second (V2); subordinate clauses use verb-final.

Why den Weg and not dem Weg?

Because sehen takes a direct object in the accusative. Weg is masculine:

Nominative: der Weg
Accusative: den Weg So: wir sehen den Weg (not: dem Weg).

What does absichtlich mean, and are there alternatives?

absichtlich = intentionally, on purpose. Common alternatives:

mit Absicht (neutral)
bewusst (consciously; often similar)
extra (colloquial: “on purpose/deliberately”; beware it can also mean “especially”)
vorsätzlich (legal/formal: premeditated; stronger than everyday “intentional”)

Where can I place absichtlich in the sentence?

Most natural placements:

Sie ließ das Licht absichtlich an. (very common)
Sie ließ absichtlich das Licht an. (focus on the intention) End-position after an is unusual: … an, absichtlich. sounds tacked-on. Rule of thumb: manner adverbs like absichtlich usually appear before the predicate complement and after the object.

Can I use weil or (so) dass instead of damit?

They don’t mean the same:

weil gives a reason/cause: Sie ließ das Licht an, weil es dunkel war.
(so) dass/sodass gives a result: Sie ließ das Licht an, sodass wir den Weg sehen konnten.
damit gives an intended purpose/goal: … damit wir den Weg sehen.

Is a comma required before damit? Can the damit-clause come first?

Yes, you must put a comma before a damit-clause.
You can front the purpose clause:
- Damit wir den Weg sehen, ließ sie das Licht absichtlich an.

Does Sie here mean “she,” “they,” or formal “you”? How can I tell?

It must be “she,” because the verb is 3rd person singular (ließ).

“they” would be sie ließen
formal “you” would also be Sie ließen So Sie ließ … = “She left …”

Why das Licht (neuter)? Could I omit the article?

Licht is neuter (das Licht). In this fixed expression, the definite article is standard: das Licht anlassen/ausmachen. Omitting the article (Sie ließ Licht an) is not idiomatic here.

What’s the difference between Weg and weg?

der Weg (capital W) = the path/way; pronounced with a long vowel: [veːk].
weg (lowercase) = away/gone; pronounced with a short vowel: [vɛk]. In the sentence, den Weg is the noun “the way/path.”

Are there alternative verbs for turning/keeping lights on or off?

Yes:

Turn on: anmachen, einschalten (Ich mache das Licht an. / Ich schalte das Licht ein.)
Turn off: ausmachen, ausschalten
Keep on/leave on: (an)lassen or an lassen (as discussed), also idiomatic: brennen lassen (leave burning): Sie ließ das Licht brennen.