Polus in sphaera videtur.

Polus is nominative singular. It is the subject of the sentence: the pole is the thing that is seen.

This is the basic idea:

• polus = the pole
• videtur = is seen

So polus has to be in the nominative because it is the subject of the verb.

Because the sentence is passive, not active.

• videt = he/she/it sees
• videtur = he/she/it is seen

Here the pole is not doing the seeing. The pole is the thing being seen. That is why Latin uses the passive form videtur.

Yes, very often videtur can mean seems. That is a very common meaning in Latin.

But in this sentence, the straightforward meaning is the literal passive:

• videtur = is seen

A learner has to use context to decide. Since you already know the meaning here, this is the ordinary passive use of video.

Because it follows the preposition in meaning in or on, and here in shows location, not motion.

With in, Latin usually works like this:

• in + ablative = in/on somewhere
• in + accusative = into/onto somewhere

So:

• in sphaera = in/on the sphere
• in sphaeram = into the sphere

Since the meaning is location, sphaera is ablative singular.

Latin in with the ablative can cover both in and on, depending on context.

So in sphaera could be understood as:

• in the sphere
• on the sphere

In English, the best choice depends on what kind of sphere is meant. If this is an astronomical or geometrical context, English may prefer one or the other, but the Latin itself simply uses in.

Because Latin has no definite article and no indefinite article.

So Latin does not have separate words for:

• the
• a
• an

That means polus can mean:

• the pole
• a pole

And sphaera can mean:

• the sphere
• a sphere

English has to add the or a based on context.

Not as much as in English. Latin word order is fairly flexible because the endings show the grammar.

Here:

• Polus = subject
• in sphaera = prepositional phrase
• videtur = verb

This order is very normal, especially with the verb at the end. But Latin could rearrange the words for emphasis, for example:

• In sphaera polus videtur

That would still mean the same basic thing.

The verb comes from video, videre, vidi, visus, meaning see.

In this sentence, videtur is:

• 3rd person singular
• present tense
• indicative mood
• passive voice

So it means he/she/it is seen.

Because the subject is polus, the translation is the pole is seen.

Because its subject, polus, is singular.

Latin verbs agree with their subjects in person and number. Since polus is one pole, the verb must also be singular:

• polus ... videtur = the pole ... is seen

If the subject were plural, the verb would be plural too.

• polus is a second-declension masculine noun.
• sphaera is a first-declension noun.

So their forms here are:

• polus = nominative singular
• sphaera = ablative singular after in

Knowing the declension helps you recognize why the endings are different.

The sentence breaks down like this:

• Polus = subject
• in sphaera = prepositional phrase showing location
• videtur = passive verb

So the pattern is:

subject + prepositional phrase + verb

That is a very common and useful Latin sentence pattern.