Puer in arborem ascendit, ut nidum videat.

Puer is nominative singular, so it is the subject of the sentence.

It belongs to the second declension, but not all second-declension masculine nouns end in -us. Some end in -er, and puer, pueri is one of them.

So:

• puer = nominative singular, the boy
• pueri = genitive singular, of the boy, or nominative plural, the boys

Because Latin uses:

• in + accusative for motion into / onto / toward
• in + ablative for location in / on

Here the boy is moving into or up into the tree, so Latin uses in arborem.

Compare:

• in arborem ascendit = he climbs into/up into the tree
• in arbore est = he is in the tree

This is a very common Latin pattern.

Arborem is accusative singular of arbor, arboris, meaning tree.

A learner can spot this because:

• arbor is a third-declension noun
• many third-declension nouns have -em in the accusative singular

So:

• arbor = nominative singular
• arborem = accusative singular

It is accusative here because in takes the accusative when it shows motion.

Here ut means so that or in order that.

It introduces a purpose clause: it explains why the boy climbs into the tree.

So the structure is:

• main action: Puer in arborem ascendit
• purpose: ut nidum videat

Latin uses ut very often for purpose clauses.

Because a purpose clause with ut normally takes the subjunctive, not the indicative.

So:

• videt = he sees or he is seeing
• videat = that he may see / so that he may see

In English, we often just use to see or so that he can see, but Latin commonly uses ut + subjunctive.

Because Latin usually does not use a bare infinitive of purpose the way English does.

English says:

• He climbs the tree to see the nest

Latin more naturally says:

• He climbs into the tree so that he may see the nest
• ut nidum videat

So even though English uses to see, Latin prefers a purpose clause with ut + subjunctive.

This is a sequence of tenses question.

In a purpose clause:

• after a primary tense, Latin usually uses the present subjunctive: videat
• after a secondary/historical tense, Latin usually uses the imperfect subjunctive: videret

The interesting thing is that ascendit can be spelled the same whether it means:

• he climbs (present), or
• he climbed / has climbed (perfect)

So:

• if ascendit is understood as he climbs, then videat is exactly what you expect
• if it is understood as a simple past narrative he climbed, many classical authors would more often use videret

That means videat strongly suggests a present-time or non-historical reading here.

Because nidum is the direct object of videat.

The verb video takes an accusative object: you see something.

So:

• videat nidum = may see the nest

Also, nidum is the accusative singular of nidus, nidi, a second-declension masculine noun.

It can mean either, because the present and perfect third-person singular forms are often spelled the same in Latin.

So ascendit may be:

• he climbs
• he climbed
• sometimes he has climbed, depending on context

Context tells you which meaning is intended. In this sentence, videat makes he climbs the most likely interpretation in standard textbook Latin.

Yes. Latin word order is much more flexible than English word order because the endings show each word’s job.

This sentence could also be arranged differently, for example:

• Puer ascendit in arborem, ut nidum videat

The given order puts in arborem before ascendit, which can make the destination feel prominent. The purpose clause naturally comes after the main clause.

So the word order is normal, but it is not the only possible order.

Because Latin has no articles.

So puer can mean:

• the boy
• a boy

and nidum can mean:

• the nest
• a nest

The context decides which English article is best. This is completely normal in Latin.