Mater puerum mane excitat, et puer statim tunicam puram induit.

Because puerum is the direct object of excitat.

• mater = the subject, the one doing the action
• puerum = the person being woken up

In Latin, the direct object usually goes in the accusative case.
The accusative singular of puer is puerum.

So:

• puer = boy as a subject
• puerum = boy as an object

That is why Latin says Mater puerum mane excitat rather than Mater puer mane excitat.

Mater is nominative singular, because it is the subject of the verb excitat.

Latin does not always mark the nominative singular with an obvious ending. Different nouns decline differently.

Here:

• mater = nominative singular, mother
• matrem = accusative singular, mother as direct object

So mater does have the right form already; it just happens that this noun’s nominative singular does not end in something very distinctive.

Because mane is an adverb, not a noun with a preposition.

It simply means in the morning or early in the morning.

So:

• mane excitat = wakes up ... in the morning

English often uses a prepositional phrase where Latin uses a single adverb.

Latin word order is much more flexible than English word order because the endings show what each word is doing.

A very common Latin pattern is to place the verb near the end of the clause. So:

• Mater puerum mane excitat

is a very natural Latin order.

But Latin could rearrange the words for emphasis and still keep the same basic meaning, for example:

• Mane mater puerum excitat
• Puerum mater mane excitat

The endings still tell you that mater is the subject and puerum is the object.

In the second clause, puer is the subject of induit:

• et puer statim tunicam puram induit

Here the boy is the one doing the action, so Latin uses the nominative:

• puer = the boy as subject

If the boy were the direct object, Latin would use puerum.

So the sentence changes from:

• mother wakes the boy
  to
• the boy puts on a clean tunic

That is why the form changes.

Because puram is an adjective describing tunicam, and in Latin adjectives must agree with the nouns they describe in:

• gender
• number
• case

Here:

• tunicam = accusative singular feminine
• puram = accusative singular feminine

They match because they belong together: a clean tunic.

This agreement is one of the most important features of Latin grammar.

In Latin, adjectives can come before or after the noun. Both are common.

So:

• tunicam puram
• puram tunicam

can both mean a clean tunic.

Latin word order often depends on style, emphasis, and rhythm more than strict rules like in English.
A learner should not assume that adjective + noun order must always match English.

Because Classical Latin has no articles.

English distinguishes:

• a boy
• the boy

Latin usually just says puer, and the exact meaning depends on context.

So:

• mater can mean mother, a mother, or the mother
• puer can mean boy, a boy, or the boy
• tunicam puram can mean a clean tunic or the clean tunic

When translating into English, you choose a/an or the based on what sounds natural in context.

Because Latin often leaves pronouns out when they are not needed.

The verb ending already tells you the person and number:

• excitat = he/she/it wakes
• induit = he/she/it puts on

Then the nouns in the sentence make it clear who is doing the action:

• mater ... excitat = the mother wakes ...
• puer ... induit = the boy puts on ...

Latin usually includes pronouns such as is, ea, or ille only when it wants extra emphasis or clarity.

In form, induit can be ambiguous, because it can represent either:

• present: puts on
• perfect: put on / has put on

In this sentence, the context strongly suggests the present tense:

• Mater ... excitat, et puer ... induit
• Mother wakes the boy ..., and the boy immediately puts on ...

The first verb excitat is clearly present, so the second is naturally understood as present too.

In a text with macrons, the distinction could be shown by vowel length, but many Latin texts do not mark that.

Here induit means puts on or dresses himself in.

The verb is induere, which often means to put something on the body, such as clothing.

So:

• tunicam puram induit = he puts on a clean tunic

Notice that the clothing item is the direct object:

• tunicam = the thing being put on

English sometimes uses get dressed, but Latin here is more specific: it tells you exactly what garment he puts on.

Statim is an adverb meaning immediately, at once, or right away.

It modifies the verb induit:

• puer statim tunicam puram induit
• the boy immediately puts on a clean tunic

Like many Latin adverbs, it can move around fairly freely without changing the basic meaning. Its position here is natural and clear.