La foto estas pruvo de la difekto.

Because -o is the normal noun ending in Esperanto.

• foto = a photo / photo
• pruvo = a proof / proof
• difekto = a defect / defect

If you see a word ending in -o, it is usually a noun.

La is the definite article, meaning the.

It is used separately before each noun that is definite:

• la foto = the photo
• la difekto = the defect

Esperanto has only one article, la, and it does not change for gender, number, or case.

Esperanto normally has no indefinite article. There is a word for the (la), but no separate word for a/an.

So:

• foto can mean a photo or just photo, depending on context.
• la foto specifically means the photo.

Estas is the present-tense form of esti, which means to be.

So:

• La foto estas pruvo... = The photo is proof...

A helpful feature of Esperanto is that estas stays the same for all persons:

• mi estas = I am
• vi estas = you are
• li/ŝi estas = he/she is
• ili estas = they are

There is no change like am / is / are in English.

Because after estas, pruvo is not a direct object. It is a predicate noun or subject complement.

In other words, the sentence is identifying what the photo is:

• La foto = subject
• estas = is
• pruvo = proof

Esperanto normally does not use -n on predicate nouns after esti.

So La foto estas pruvon... would be wrong here.

Here de means of.

So:

• pruvo de la difekto = proof of the defect

In Esperanto, de often shows a relationship like of, from, or possession/connection, depending on context.

Here it links proof to the thing being proved.

Normally, de is the better choice here.

• pruvo de la difekto = proof of the defect

Pri usually means about or concerning, so pruvo pri la difekto would sound more like proof about the defect, which is less natural if you mean the photo proves that the defect exists.

So for this sentence, de is the expected preposition.

Because the sentence is talking about the defect as a thing, not describing something as defective.

• difekto = defect
• difekta = defective

So:

• la difekto = the defect
• la difekta foto = the defective photo

In your sentence, the photo is proof of the defect, so the noun difekto is needed.

The normal word order here is very natural:

• La foto estas pruvo de la difekto.

Esperanto word order is more flexible than English, but this standard order is the clearest for learners.

You could change the order for emphasis in some contexts, but this version is the straightforward, neutral one.

Yes. Both exist.

• foto = photo
• fotografio = photograph / photography, depending on context

Foto is very common and natural in everyday Esperanto. In this sentence, la foto is completely normal.

A simple guide is:

La FO-to ES-tas PRU-vo de la di-FEK-to.

A few helpful pronunciation points:

• a sounds like ah
• e sounds like eh
• i sounds like ee
• o sounds like oh
• u sounds like oo

Stress in Esperanto usually falls on the second-to-last syllable:

• FO-to
• ES-tas
• PRU-vo
• di-FEK-to

Yes. La difekto suggests that the defect is specific or already identifiable from context.

For example, maybe both speaker and listener already know which defect is being discussed.

If you wanted a more general idea, you might say just difekto without la, but in this sentence la difekto naturally means the particular defect.

Yes. Because Esperanto has no indefinite article, pruvo can often correspond to English a proof or just proof, depending on context.

In English, we often say The photo is proof of the defect rather than The photo is a proof of the defect, but Esperanto pruvo itself does not force one exact English article choice.

So the Esperanto grammar is simple here; the most natural English wording depends on style.