Serva calcem aqua tepida fovet, quo puella commodius ambulare possit.

Because serva is in the nominative singular, which is the usual case for the subject of a finite verb.

• serva = the slave woman / maidservant
• fovet = she warms / soothes

So serva fovet means the maidservant soothes.

Because it is the direct object of fovet.

The verb fovet takes an object: someone or something is being warmed, soothed, or treated. Here that thing is calcem.

• dictionary form: calx, calcis = heel
• accusative singular: calcem

So calcem fovet means she soothes the heel.

Literally, fovet means warms, keeps warm, cherishes, or soothes, depending on context.

In this sentence, with aqua tepida, it has a practical, physical sense:

• she warms the heel with warm water
• or more naturally in English, she bathes / soothes / treats the heel with warm water

So the idea is not just abstract comfort, but actual physical care.

This is a very common use of the ablative of means or instrument.

Latin often uses the ablative without a preposition to show the means by which something is done.

• aqua = ablative singular of aqua
• tepida agrees with aqua, so it is also ablative singular

So aqua tepida means with warm water.

This is like:

• gladio pugnat = he fights with a sword
• litteris scribit = he writes with letters / in writing

Here quo introduces a purpose clause: so that, in order that.

A learner often first meets purpose clauses with ut, but Latin very often uses quo instead when the clause contains a comparative, especially an adverb like commodius.

So:

• ut puella commode ambulare possit = so that the girl may be able to walk comfortably
• quo puella commodius ambulare possit = so that the girl may be able to walk more comfortably

Because commodius is comparative, quo is especially natural here.

Commodius is a comparative adverb.

It comes from commode = comfortably / conveniently / suitably.

So:

• commode = comfortably
• commodius = more comfortably

It modifies ambulare:

• ambulare commodius = to walk more comfortably

This comparative adverb is one reason Latin uses quo in the purpose clause.

Because it is in a purpose clause, and purpose clauses in Latin normally take the subjunctive.

So:

• potest = is able / can (indicative)
• possit = may be able / can in a purpose clause (present subjunctive)

The structure is:

• quo ... possit = so that ... may be able

This is normal Latin grammar after ut or quo when expressing purpose.

Because possum regularly takes a complementary infinitive.

So:

• potest ambulare = she can walk
• possit ambulare = she may be able to walk

The infinitive ambulare completes the meaning of possit.

English does something similar:

• she is able to walk
• she can walk

Grammatically, serva and puella are simply two different nouns:

• serva = the maidservant, subject of fovet
• puella = the girl, subject of possit

So the most straightforward reading is that the maidservant is treating the girl's heel so that the girl can walk more comfortably.

They are probably different people, though only context can make that absolutely certain. Because Latin explicitly names puella in the subordinate clause, the sentence naturally points to her as the person who will benefit.

Latin often leaves relationships to be understood from context when they are obvious enough.

Here the sentence says:

• the maidservant soothes the heel
• so that the girl may walk more comfortably

That strongly suggests the heel belongs to the girl, even though Latin does not add a possessive word.

A more explicit version could have used something like a possessive adjective or genitive, but Latin often prefers the shorter form when the meaning is clear from the situation.

Latin word order is flexible, so the grammar is shown mainly by endings, not by position.

The order here is perfectly natural:

• Serva — subject first
• calcem — object
• aqua tepida — means/instrument
• fovet — verb at the end of the main clause
• quo puella commodius ambulare possit — purpose clause after it

A very literal unpacking would be:

• The maidservant the heel with warm water soothes, so that the girl may be able to walk more comfortably.

The order helps the sentence flow, but the endings tell you who is doing what.

Possit is present subjunctive, third person singular.

It is present because the purpose is viewed as simultaneous with or following from the main action:

• the maidservant is treating the heel
• so that the girl may be able to walk more comfortably

This is the normal tense for a purpose clause after a present-tense main verb like fovet.

Historically, that is close to where this use comes from, but in this sentence it is best understood simply as so that or in order that.

A very literal sense like by which would sound awkward in normal English. For a learner, the safest approach is:

• quo
  • comparative + subjunctive = usually so that ... more ...

So here:

• quo ... commodius ... possit = so that ... may be able to walk more comfortably