Labrum nondum desinit tumere, sed puer iam sine dolore bibere potest.

Labrum is nominative singular neuter.

Why nominative? Because it is the subject of desinit: the lip is the thing that has not yet stopped swelling.

A few useful details:

• labrum, labrī = a second-declension neuter noun
• in neuter second-declension nouns, the nominative singular and accusative singular look the same
• here the meaning and the verb show that labrum is the subject, so it is nominative

Also, even though it is neuter, it still takes a singular verb: desinit.

Because Latin often uses desinō + infinitive to mean stop doing something.

So:

• desinit = he/she/it stops
• tumere = to swell
• desinit tumere = stops swelling

This is very similar to English stop + -ing, though Latin uses an infinitive rather than a gerund here.

Tumere is the present active infinitive of tumeō, meaning to swell.

So the basic parts are:

• tumeō = I swell / am swollen
• tumēre = to swell

In this sentence, it depends on desinit:

• labrum nondum desinit tumere = the lip has not yet stopped swelling

Nondum means not yet.

It is an adverb, and it modifies desinit:

• nondum desinit = has not yet stopped

A learner may notice that English often puts not yet near the end, but Latin can place nondum before the verb very naturally.

Because puer is the subject of potest.

The second clause is:

• sed puer iam sine dolore bibere potest

Here:

• puer = the boy
• potest = is able / can

So the boy is the one who can drink, which makes puer the subject and therefore nominative.

Because possum normally takes an infinitive in Latin.

So:

• potest = he can
• bibere = to drink
• bibere potest = he can drink

This is one of the most common Latin patterns:

• possum + infinitive = can / be able to do something

English often uses can drink, while Latin uses can + to drink only in form, not in translation. You should not translate bibere separately as to drink in a stiff way every time; the whole phrase naturally means can drink.

Because the preposition sine takes the ablative case.

So:

• sine = without
• dolor, dolōris = pain
• dolore = ablative singular

Together:

• sine dolore = without pain

This is a standard prepositional phrase, and the ablative is required by sine.

They create a contrast:

• nondum = not yet
• iam = already / now

So the sentence sets up a clear comparison:

• the lip has not yet stopped swelling
• but the boy already can drink without pain

That contrast is reinforced by sed = but.

Latin word order is much more flexible than English word order because Latin shows grammatical relationships mostly through endings, not position.

So even though the sentence is arranged as:

• Labrum nondum desinit tumere
• sed puer iam sine dolore bibere potest

the meanings are still clear because of the forms:

• labrum and puer are subjects
• tumere and bibere are infinitives
• dolore is ablative after sine

Latin word order often helps with emphasis and flow, not just grammar.

Yes, iam can mean either now or already, depending on context.

In this sentence, already is usually the most natural choice, because it contrasts well with nondum:

• the swelling has not yet stopped
• but the boy can already drink without pain

So already brings out the sense of improvement before complete recovery.

It can mean lip, but a learner may notice that labium is also a common Latin word for a bodily lip.

Very broadly:

• labrum can mean a lip, edge, or rim
• labium is often the more straightforward word for a lip of the mouth

So if you were wondering about the vocabulary choice, that is a fair question. In this sentence, though, labrum clearly means lip, and the grammar works normally.

The two finite verbs are:

• desinit = 3rd person singular present active indicative of desinō
• potest = 3rd person singular present active indicative of possum

Why singular?

• labrum is singular, so desinit is singular
• puer is singular, so potest is singular

The infinitives attached to them are:

• tumere with desinit
• bibere with potest

So the sentence has two coordinated clauses, each with a singular subject and a finite verb.