Fideiussor pro debitore spondet.

• fideiussor: the subject of the sentence, in the nominative singular
• pro: a preposition
• debitore: the object of the preposition pro, in the ablative singular
• spondet: the verb, 3rd person singular present active indicative

So structurally, the sentence is:

[Subject] + [prepositional phrase] + [verb]

Because pro takes the ablative in Latin.

So:

• debitor = debtor
• debitore = by/for/on behalf of the debtor, depending on context

This is something English speakers often have to get used to: in Latin, the preposition controls the case of the noun that follows it.

Because debitorem would be the accusative singular, but pro does not take the accusative here. It takes the ablative.

So the contrast is:

• debitor = nominative
• debitorem = accusative
• debitore = ablative

Since the sentence has pro, the correct form is pro debitore.

spondet is:

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

It comes from spondeo, spondere.

So spondet means he/she/it promises or pledges.

Because the subject is fideiussor, the sense is the guarantor promises.

Because Latin usually does not need an expressed subject pronoun when the verb ending already shows the person and number.

The ending -t in spondet already tells you it is 3rd person singular.

Also, the noun fideiussor is already there as the subject, so adding a pronoun would be unnecessary unless you wanted special emphasis.

Latin has no articles like English the or a/an.

So fideiussor can mean:

• a guarantor
• the guarantor

And debitore can mean:

• for a debtor
• for the debtor

Which one is meant depends on the context, not on a separate article.

Here pro means something like:

• for
• on behalf of
• in place of

In this sentence, it shows the person for whom the guarantor is making the promise.

So pro debitore means for the debtor or on behalf of the debtor.

In a legal context, that is especially important: the guarantor is undertaking an obligation connected with the debtor.

fideiussor is a 3rd-declension noun, nominative singular here.

It is the dictionary form of the noun, and in this sentence it is the subject.

A learner may also notice that it is a fairly technical word: it means a guarantor or surety, especially in legal language.

Yes. Latin word order is more flexible than English word order because the grammatical endings show the relationships between words.

So all of these could express basically the same idea:

• Fideiussor pro debitore spondet
• Pro debitore fideiussor spondet
• Spondet fideiussor pro debitore

The basic meaning stays the same because:

• fideiussor is still nominative
• debitore is still ablative after pro
• spondet is still the verb

However, changing the order can change the emphasis.

Not always. spondere can mean to promise, but it often has a formal or even legal flavor.

In a sentence like this, with fideiussor and debitor, it is very natural to understand spondet as a formal pledge or legal undertaking, not just a casual promise.

So an English speaker should not imagine something as light as I promise I’ll call you later. This is much more serious and technical.

Because pro debitore is a prepositional phrase, and in this sentence it most naturally tells you in relation to whom the action of spondet is being performed.

In other words:

• the fideiussor is the subject
• spondet is the action
• pro debitore explains for whom / on whose behalf

So it functions adverbially with the verb, even though in English we may naturally connect the whole idea closely with guarantor. Latin often leaves that relationship to be understood from the structure and context.