In alta quercu aquila videri potest.

Because in takes the ablative when it means location and answers where?

So in alta quercu means in / on / up in a tall oak.

If there were motion into the tree, Latin would normally use the accusative instead:

• in alta quercu = in a tall oak, in the tall oak
• in altam quercum = into a tall oak, into the tall oak

Here the eagle is being located, not moving, so the ablative is correct.

Alta agrees with quercu in gender, number, and case.

• quercus is feminine
• here it is singular
• with in of location, it is ablative

So the adjective must also be feminine singular ablative.

A useful detail: if you write Latin without macrons, the feminine nominative singular alta and the feminine ablative singular altā look the same. In a fully marked text, the phrase would be in altā quercū.

Videri is the present passive infinitive of videre, meaning to see.

So:

• videre = to see
• videri = to be seen

That is why videri potest means can be seen or is able to be seen.

Latin often expresses can + verb by using a form of posse with an infinitive.

So here:

• potest = can, is able
• videri = to be seen

Together:

• videri potest = can be seen

This works just like:

• videre potest = he, she, it can see
• videri potest = he, she, it can be seen

Because the sentence is passive.

In an active sentence, the eagle would be the object, so it would be accusative:

• aliquis aquilam videre potest = someone can see the eagle

But in the passive sentence, the eagle becomes the subject:

• aquila videri potest = the eagle can be seen

So aquila is nominative because it is the subject of the passive idea.

Yes. Latin word order is much more flexible than English word order.

All of these are possible Latin arrangements:

• In alta quercu aquila videri potest
• Aquila in alta quercu videri potest
• Aquila videri potest in alta quercu

The sentence you were given begins with in alta quercu to set the scene first: in the tall oak...

That kind of fronting is very common in Latin.

In practice, it means the eagle is in / on / up in the tall oak.

Latin in with the ablative can cover several English spatial ideas depending on context. With a tree, English often says:

• in the tree
• up in the tree

Latin uses in alta quercu quite naturally for that idea.

There is no separate word for the or a in Latin. Latin has no articles.

So:

• aquila can mean an eagle or the eagle
• quercu can mean an oak or the oak

Which English article you choose depends on the context and the intended meaning.

No. Quercus is feminine.

This is a good reminder that Latin noun endings do not always tell you the gender reliably. Many nouns ending in -us are masculine, but not all.

Also, quercus belongs mainly to the fourth declension, which is why you get forms like quercu.

So in this sentence:

• quercus = oak, oak tree
• quercu = in the oak, on the oak

Because videtur can be tricky.

It can mean:

• is seen
• but very often also seems or appears

So aquila videtur might easily be understood as the eagle seems or the eagle appears.

By using videri potest, Latin clearly expresses can be seen and avoids that ambiguity.