La photo datée montre la dégradation.

Here, datée means dated in the sense of having a date on it or being assigned to a particular date.

So la photo datée means something like the dated photo or the photo with a date on it.

Be careful: in other contexts, daté / datée can also mean old-fashioned or outdated, but that is not the most natural reading here.

Because datée agrees with photo, which is feminine singular.

Agreement here works like an adjective:

• masculine singular: daté
• feminine singular: datée
• masculine plural: datés
• feminine plural: datées

Since la photo is feminine singular, French uses datée.

In French, most adjectives come after the noun, and this is one of those cases.

So French says:

• la photo datée

not usually:

• la datée photo

That sounds wrong in normal French.

It is originally the past participle of dater (to date), but in this sentence it is functioning like an adjective describing photo.

This is very common in French. A past participle can be used adjectivally, just like in English:

• a broken window
• une fenêtre cassée

So here:

• une photo datée = a dated / date-marked photo

French often uses the present tense to describe what a picture, map, document, or chart shows.

So:

• La photo montre... = The photo shows...

This is completely natural, just like in English.

Montre is the third-person singular present form of montrer, because the subject is la photo.

Dégradation often means deterioration, damage, or wearing down.

Depending on context, it can refer to:

• physical deterioration of a building or object
• worsening conditions
• damage or decay

So in this sentence, it probably means that the photo shows some kind of damage or deterioration.

French uses articles more often than English does. In many cases, a noun needs an article even where English might be more flexible.

So la dégradation can mean:

• the deterioration
• the damage
• the degradation

It suggests that the deterioration is being treated as a specific thing visible in the photo, or as the phenomenon under discussion.

Grammatically, datée can sometimes mean outdated or old-fashioned, so that interpretation is not impossible in isolation.

But in this sentence, the more likely meaning is dated in the literal sense of bearing a date or coming from a known date.

Context would decide. If this appeared in a historical, scientific, or documentary context, photo datée would usually be understood as dated photo.

Yes, punctuation can slightly change how the sentence feels.

• La photo datée montre la dégradation
  = the dated photo shows the deterioration

• La photo, datée, montre la dégradation
  = the photo, which is dated, shows the deterioration

Without commas, datée is more tightly attached to photo as a defining description. With commas, it sounds more like extra information.

A simple pronunciation guide is:

La photo datée montre la dégradation
la fo-to da-tay montr la day-gra-da-syon

A few helpful points:

• photo: the ph sounds like f
• datée: the final -ée sounds like ay
• montre: the final e is not pronounced
• dégradation: the ending -tion sounds like syon

In natural speech, French flows smoothly, so the whole sentence sounds connected rather than word-by-word.