Learn Hardcore German: Ich lege die Stirnlampe in den Rucksack.

legen – to lay something down flat / in a lying position
- Ich lege das Buch auf den Tisch.
  I lay/put the book (flat) on the table.
stellen – to put something upright / in a standing position
- Ich stelle die Flasche auf den Tisch.
  I put the bottle (standing) on the table.
setzen – to set something (often for people sich setzen = to sit down)
- Ich setze das Kind auf den Stuhl.
  I set the child on the chair.

For a headlamp (Stirnlampe), legen is natural because it’s a small object being laid/placed into something.

You could also hear Ich packe die Stirnlampe in den Rucksack (I pack the headlamp into the backpack), which focuses more on packing rather than the exact position.

Why is it “die Stirnlampe” and not “der” or “das Stirnlampe”?

Every German noun has a grammatical gender: masculine (der), feminine (die), or neuter (das).

Stirnlampe is grammatically feminine, so its article in the nominative case is die:
die Stirnlampe – the headlamp

There is no rule you can apply perfectly to guess the gender; you usually have to learn it together with the noun:

der Rucksack – the backpack (masculine)
die Lampe – the lamp (feminine)
das Auto – the car (neuter)

So you should store it in your memory as die Stirnlampe.

Why does “die Stirnlampe” stay “die” here? Isn’t that accusative?

Yes, die Stirnlampe is in the accusative case because it is the direct object (the thing being put).

For feminine nouns, the article is the same in nominative and accusative:

Nominative: die Stirnlampe liegt auf dem Tisch.
Accusative: Ich lege die Stirnlampe in den Rucksack.

Compare with masculine nouns, where nominative and accusative are different:

Nominative: der Rucksack ist schwer.
Accusative: Ich trage den Rucksack.

Why is it “in den Rucksack” and not “in dem Rucksack”?

In is a so‑called two-way preposition in German. It can take:

Accusative – if there is movement into something (direction)
Dative – if something is already located somewhere (no movement)

In this sentence, there is movement into the backpack:

Ich lege die Stirnlampe in den Rucksack.
→ motion into: accusative (masculine) → den Rucksack

If you describe a static location, you use dative:

Die Stirnlampe ist in dem Rucksack.
(The headlamp is in the backpack.)

Spoken German often contracts in dem to im:

Die Stirnlampe ist im Rucksack.

Why “den Rucksack” and not “der Rucksack”?

Rucksack is masculine:

Nominative singular: der Rucksack
Accusative singular: den Rucksack

After in with movement, we use the accusative, so the article changes:

Nominative: der Rucksack ist neu.
Accusative: Ich lege etwas in den Rucksack.

That’s why we say in den Rucksack, not in der/derm Rucksack.

What is the grammatical role of “die Stirnlampe” and “in den Rucksack” in this sentence?

In Ich lege die Stirnlampe in den Rucksack:

die Stirnlampe = direct object (accusative)
→ What am I laying/putting? The headlamp.
in den Rucksack = prepositional phrase with in + accusative expressing direction
→ Where to? Into the backpack.

Verb pattern:
legen = jemand legt etwas irgendwohin
(someone lays something somewhere (to some place))

Could I also say “Ich lege in den Rucksack die Stirnlampe”? Is that wrong?

It’s grammatically correct, but unusual in everyday speech and can sound stylistically marked or poetic.

Normal, neutral order:

Ich lege die Stirnlampe in den Rucksack.

Alternative orders are used only for special emphasis or rhythm, for example in written or literary German. In general, stick to:

Subject – verb – (direct object) – (prepositional phrase)
→ Ich lege die Stirnlampe in den Rucksack.

Is “Rucksack” exactly the same as English “backpack”?

Mostly yes:

der Rucksack = the backpack / rucksack

It’s the usual word for what you carry on your back with two straps.

Some related words:

die Tasche – bag (can be handbag, shoulder bag, etc.)
die Reisetasche – travel bag / duffel bag
der Schulrucksack – school backpack

So in most contexts, you can translate Rucksack as backpack.

Does German present tense here mean “I am putting”? There’s no separate continuous form?

Correct. German does not have a special continuous tense like English “I am putting”.

The present tense (ich lege) can express:

a current action:
Ich lege die Stirnlampe in den Rucksack.
= I am putting the headlamp into the backpack.
a general habit / repeated action
a near future (in some contexts)

So Ich lege … can usually be translated as either “I put” or “I am putting”, depending on context.

Could I use tun instead of legen, like “Ich tue die Stirnlampe in den Rucksack”?

You will hear sentences like:

Ich tue die Stirnlampe in den Rucksack.

This is common in some dialects and in very informal speech, but in standard German it’s better to use a more specific verb:

Ich lege die Stirnlampe in den Rucksack. – neutral, standard
Ich packe die Stirnlampe in den Rucksack. – focus on packing

The verb tun is very generic (to do/put) and is often avoided in careful written German in this sense.