Unser Sohn geht seit diesem Monat in die Krippe.

Because Sohn is masculine singular and it is the subject of the sentence, so it is in the nominative case.

With possessive words like mein, dein, unser, the nominative forms are:

• unser Sohn (masculine)
• unsere Tochter (feminine)
• unser Kind (neuter)
• unsere Kinder (plural)

So unser Sohn is the correct nominative masculine form.

In German, all nouns are capitalized, not just proper names.

So in this sentence:

• Sohn is a noun
• Monat is a noun
• Krippe is a noun

That is why all three begin with a capital letter.

German often uses the present tense where English might use either:

• the simple present: goes
• or the present perfect/progressive idea: has been going

This is especially common with seit (since), which refers to something that started in the past and still continues now.

So German says:

• Er geht seit diesem Monat ...

Where English often prefers something like:

• He has been going ... since this month
• or He started going ... this month

So the German present tense here is completely normal.

Because seit always takes the dative case.

The basic form is:

• dieser Monat = this month (nominative)

After seit, it changes to dative:

• seit diesem Monat

Since Monat is masculine, the dative singular form of dieser is diesem.

So:

• seit + dative
• seit diesem Monat

Because the phrase is in the dative, not the accusative.

For the masculine singular forms of dieser, compare:

• dieser = nominative
• diesen = accusative
• diesem = dative

Since seit requires the dative, you get:

• seit diesem Monat

If you used diesen Monat, that would be accusative, so it would not fit after seit.

Because in is a two-way preposition. It can take either:

• accusative for direction / movement toward something
• dative for location / being inside somewhere

Here, geht in die Krippe expresses the idea of going to / starting to attend daycare, so German uses the accusative:

• in die Krippe

Compare:

• Unser Sohn geht in die Krippe. = he goes to daycare / attends daycare
• Unser Sohn ist in der Krippe. = he is in daycare / he is at the daycare right now

So die Krippe is correct because this sentence uses gehen and expresses movement or entry into that institution.

Not necessarily. In German, gehen is often used in expressions about attending institutions, not just literal walking.

For example:

• in die Schule gehen = to go to school / attend school
• in den Kindergarten gehen = to go to kindergarten
• in die Krippe gehen = to go to daycare / attend the nursery

So this phrase usually describes attendance, not the physical method of travel.

Here, Krippe means a daycare/nursery for very young children, usually younger than typical Kindergarten age.

This word can confuse learners because Krippe can also mean:

• a crib
• or a manger / nativity scene in religious contexts

But in this sentence, with Sohn and geht in die Krippe, it clearly refers to a childcare institution.

The word order is natural, but not completely fixed.

The main rule in a German main clause is that the finite verb must be in second position. Here:

• Unser Sohn = position 1
• geht = position 2

Then the other elements follow:

• seit diesem Monat
• in die Krippe

This order is very normal. But you could also say:

• Seit diesem Monat geht unser Sohn in die Krippe.

That version puts the time expression first, but geht still stays in second position.

So the sentence structure is flexible, but the verb-second rule must be respected.