Meine Enkelin baut mit einem großen Bauklotz einen Turm.

Because Enkelin is a feminine noun, and the possessive mein- has to agree with it.

• der Enkel = grandson
• die Enkelin = granddaughter

In the nominative singular feminine, mein becomes meine.

So:

• mein Enkel = my grandson
• meine Enkelin = my granddaughter

In this sentence, meine Enkelin is the subject, so it is in the nominative case.

Because the subject is meine Enkelin, which is third person singular: she.

The infinitive is bauen = to build.

Present tense:

• ich baue
• du baust
• er/sie/es baut
• wir bauen
• ihr baut
• sie/Sie bauen

So meine Enkelin baut means my granddaughter builds / is building.

Because the preposition mit always takes the dative case.

So:

• mit
  • dative

Since Bauklotz is a masculine noun (der Bauklotz), the forms become:

• ein → einem
• groß → großen

That is why you get:

• mit einem großen Bauklotz

This whole phrase means with a big building block.

Einem is the dative singular masculine form of ein.

The noun is:

• der Bauklotz = masculine

After mit, you need the dative:

• nominative: ein Bauklotz
• accusative: einen Bauklotz
• dative: einem Bauklotz

So mit einem Bauklotz is correct because mit requires dative.

This is because of adjective endings.

In mit einem großen Bauklotz:

• mit requires dative
• Bauklotz is masculine singular
• the article is einem

After an ein-word like ein/einem, the adjective in masculine dative singular takes -en.

So:

• mit einem großen Bauklotz

This is a very common pattern:

• mit einem kleinen Hund
• mit einem alten Mann
• mit einem großen Bauklotz

Because einen Turm is the direct object of the verb baut.

The noun is:

• der Turm = tower, masculine

A direct object is usually in the accusative case.
For a masculine noun with ein, the accusative form is:

• nominative: ein Turm
• accusative: einen Turm

So:

• Meine Enkelin baut einen Turm.

That is why it is einen Turm, not ein Turm.

You can tell from both meaning and case marking.

In this sentence:

• Meine Enkelin = subject, nominative
• einen Turm = direct object, accusative
• mit einem großen Bauklotz = prepositional phrase, dative

A useful clue is the article endings:

• meine Enkelin → nominative feminine subject
• einen Turm → accusative masculine object
• einem großen Bauklotz → dative after mit

So the structure is:

[Subject] Meine Enkelin
[Verb] baut
[Prepositional phrase] mit einem großen Bauklotz
[Direct object] einen Turm

German word order is flexible, but not random.

In a normal main clause, the conjugated verb goes in second position:

• Meine Enkelin baut ...

After that, German often puts adverbial information (like mit einem großen Bauklotz) before the direct object, especially if the object is indefinite.

So this sentence sounds natural:

• Meine Enkelin baut mit einem großen Bauklotz einen Turm.

You can also say:

• Meine Enkelin baut einen Turm mit einem großen Bauklotz.

That version is also grammatical, but the emphasis may feel a little different. The original sentence gives the method/instrument first, then the thing being built.

Yes. Bauen is a normal verb for building in both serious and playful contexts.

It can be used for:

• houses
• walls
• towers
• models
• things made by children with blocks

So einen Turm bauen is a very natural phrase for a child building a tower out of blocks.

Because in German, all nouns are capitalized.

In this sentence, the nouns are:

• Enkelin
• Bauklotz
• Turm

This is one of the easiest ways to spot nouns in written German.

Adjectives and verbs are not capitalized here:

• meine
• baut
• großen

Bauklotz is a compound noun, which is very common in German.

It is made from:

• Bau = building / construction
• Klotz = block

So Bauklotz literally means something like building block.

German often combines nouns into one longer word:

• Haus + Tür = Haustür
• Kinder + Zimmer = Kinderzimmer
• Bau + Klotz = Bauklotz

In this sentence, meine means my because it comes directly before a noun:

• meine Enkelin = my granddaughter

German mein- words change their endings depending on gender, case, and number:

• mein Sohn
• meine Tochter
• meinen Bruder
• meinem Freund

Used by itself, meiner / meine / meins can sometimes correspond more to mine, but that is a different structure.

A rough English-friendly guide is:

• Meine ≈ MY-nuh
• Enkelin ≈ ENG-kuh-lin
• baut ≈ bowt (rhymes roughly with out)
• mit ≈ mit
• einem ≈ EYE-nem
• großen ≈ GROH-sen
• Bauklotz ≈ BOW-klots
• einen ≈ EYE-nen
• Turm ≈ toorm

A few useful pronunciation notes:

• au usually sounds like the ow in now
• ei usually sounds like the i in my
• z in German sounds like ts
• w in German sounds like English v
• r is usually softer than in English, especially at the end of syllables