Meine Mutter legt eine Mandarine in meine Brotdose.

Because Mandarine is a feminine noun in German: die Mandarine.

In this sentence, Mandarine is the direct object of legt, so it is in the accusative case. For feminine nouns, the indefinite article stays eine in both the nominative and accusative:

• nominative: eine Mandarine
• accusative: eine Mandarine

So einen Mandarine would be wrong. Einen is used with masculine nouns in the accusative, for example einen Apfel.

Both Mutter and Brotdose are feminine singular nouns.

The possessive word mein- changes its ending depending on gender, number, and case. Here:

• meine Mutter = feminine singular, nominative
• meine Brotdose = feminine singular, accusative

For feminine singular, the form is meine in both of those cases. That is why both look the same here.

Compare:

• mein Vater = my father
• meine Mutter = my mother
• mein Brot = my bread
• meine Brotdose = my lunchbox

Because in is a two-way preposition. That means it can take either:

• accusative for movement/change of location
• dative for location/no movement

Here, the sentence describes putting something into the lunchbox, so there is movement. That is why German uses the accusative:

• in meine Brotdose = into my lunchbox

If the sentence described where the mandarin already is, you would use the dative:

• Die Mandarine liegt in meiner Brotdose. = The mandarin is in my lunchbox.

So:

• legen + in + accusative = put into
• liegen + in + dative = be lying in

Legt is the 3rd person singular present tense of legen.

• ich lege = I put / lay
• du legst = you put / lay
• er/sie/es legt = he/she/it puts / lays

In this sentence, the subject is Meine Mutter, which is she, so the verb form is legt.

Legen usually means to lay or to put something somewhere, especially when an object is placed down. It is a transitive verb, so it normally takes a direct object:

• Sie legt eine Mandarine ...
• Er legt das Buch auf den Tisch.

This is a very common German question.

• legen = to lay / to put something somewhere
  → usually involves an action done by someone
• liegen = to lie / to be lying somewhere
  → describes a position, not an action

Compare:

• Meine Mutter legt eine Mandarine in meine Brotdose.
  She is putting it in.

• Die Mandarine liegt in meiner Brotdose.
  The mandarin is lying in it.

A useful pattern:

• legen
  • accusative object + place of movement
• liegen
  • place of location

Because German main clauses usually follow the verb-second rule (V2). That means the finite verb comes in the second position.

Here the sentence is structured like this:

• Meine Mutter = position 1
• legt = position 2
• eine Mandarine in meine Brotdose = the rest

This is standard German word order in a statement.

You could also move another element to the front, but the verb would still stay second:

• In meine Brotdose legt meine Mutter eine Mandarine.

That version is possible, but less neutral.

Yes. German very often makes compound nouns, and they are written as one word.

• Brot = bread
• Dose = box, container, tin

So Brotdose literally means something like bread box, but in everyday German it usually means a lunchbox or snack box.

This is very normal in German. Other examples:

• Hausaufgabe = house + task = homework
• Kühlschrank = cool + cupboard = refrigerator
• Handschuh = hand + shoe = glove

The last part usually determines the main meaning and the gender. Since Dose is feminine (die Dose), die Brotdose is also feminine.

Because all nouns are capitalized in German.

So in this sentence:

• Mutter is a noun
• Mandarine is a noun
• Brotdose is a noun

That is why they begin with capital letters.

This is a regular rule in German, unlike English, where common nouns are not normally capitalized.

In everyday spoken German, some people do say tun like this, especially in informal speech:

• Sie tut eine Mandarine in die Brotdose.

But for learners, legen is usually the better and more precise verb here. It sounds more standard and more natural in careful German.

So:

• legen = better choice for this sentence
• tun = possible in colloquial speech, but less elegant

German often uses in by itself and lets the case show the difference between in and into.

So:

• in + accusative often means into
• in + dative often means in

That is why:

• in meine Brotdose = into my lunchbox
• in meiner Brotdose = in my lunchbox

German can also use words like hinein for extra emphasis:

• Meine Mutter legt eine Mandarine in meine Brotdose hinein.

But that is not necessary here. The original sentence is completely normal.

Yes, legt is in the present tense.

German present tense is often used not only for what is happening now, but also for scheduled or near-future actions, depending on context.

So this sentence could mean:

• your mother is putting a mandarin in your lunchbox now, or
• this is what she does / is about to do, depending on the situation

German often uses the present tense where English might choose either is putting, puts, or sometimes even will put. Context usually makes it clear.