Philosopha in bibliotheca de natura hominum loquitur.

Philosopha is the feminine form, meaning a female philosopher. It is also in the nominative singular, which is the case normally used for the subject of the sentence.

So here:

• philosopha = the philosopher (female) as the one doing the action

If the philosopher were male, Latin would usually use philosophus instead.

Philosopha is nominative singular.

A learner often identifies this from its job in the sentence:

• it is the subject
• it matches the verb loquitur (she speaks / is speaking)

In first-declension nouns, the nominative singular often ends in -a, so philosopha fits that pattern.

Great question. Loquitur comes from loquor, loqui, locutus sum, meaning to speak. This is a deponent verb.

A deponent verb:

• has passive-looking forms
• but an active meaning

So:

• loquitur looks like a passive form
• but it means she speaks or she is speaking, not she is spoken

Specifically, loquitur is:

• 3rd person singular
• present tense
• indicative mood

Because in can take different cases depending on the meaning:

• in + ablative = in / on a place, showing location
• in + accusative = into / onto a place, showing motion toward

Here the philosopher is already in the library, not moving into it, so Latin uses the ablative:

• in bibliotheca = in the library

Compare:

• in bibliotheca loquitur = she speaks in the library
• in bibliothecam intrat = she enters into the library

Here bibliotheca is ablative singular, because it follows in in the sense of location.

So:

• bibliotheca by itself could be nominative or ablative singular in form
• but after in meaning in a place, it is understood as ablative

The preposition de usually means about, concerning, from, or down from, depending on context. In this sentence it means about.

De takes the ablative case, so:

• natura is ablative singular
• de natura = about the nature

This is a very common Latin pattern:

• de amicitia = about friendship
• de bello = about the war
• de hominibus = about people

Hominum is the genitive plural of homo, hominis, meaning human being or person.

The genitive often shows a relationship like of in English. So:

• natura hominum = the nature of human beings

That is why Latin uses:

• de natura hominum = about the nature of human beings

Grammatically, hominum comes from homo, which usually means human being, person, or people in a general sense.

That is different from vir, which more specifically means man in the sense of an adult male.

So in this sentence, hominum is most naturally understood as:

• human beings
• people
• sometimes in older translations, men in the generic sense

Yes. Latin word order is much more flexible than English word order because the endings show how the words function.

This sentence is:

• Philosopha — subject
• in bibliotheca — location
• de natura hominum — topic
• loquitur — verb

Putting the verb at the end is very common in Latin. But other orders are possible, for example:

• In bibliotheca philosopha de natura hominum loquitur.
• De natura hominum philosopha in bibliotheca loquitur.

These all mean basically the same thing, though the emphasis may shift slightly.

You can tell from the grammar:

• natura = nature
• hominum = of human beings

The genitive hominum naturally depends on natura, so together they form the phrase:

• natura hominum = the nature of human beings

Then that whole phrase is governed by de:

• de natura hominum = about the nature of human beings

So the structure is:

• de
  • natura hominum

not

• de natura and separately hominum

The dictionary form is loquor.

More fully, a dictionary may give:

• loquor, loqui, locutus sum

This tells you it is a deponent verb. From that, you can recognize:

• loquitur = he/she speaks, is speaking

So in this sentence, because the subject is philosopha, it means:

• the female philosopher speaks

Both the subject and the verb would change.

Singular:

• Philosopha ... loquitur.
• The philosopher ... speaks.

Plural:

• Philosophae ... loquuntur.
• The philosophers ... speak.

So:

• philosopha → philosophae
• loquitur → loquuntur

This shows the normal agreement between subject and verb in number.