Yağmur planı mahvetti.

Because planı is the definite direct object form.

In Turkish, a specific direct object often takes the accusative ending:

• plan = plan
• planı = the plan / that specific plan as the object

So in Yağmur planı mahvetti, the speaker means a particular plan was ruined.

A useful contrast:

• Yağmur planı mahvetti = ...ruined the plan
• Yağmur bir plan mahvetti = ...ruined a plan

Since plan ends in a consonant, the suffix is added directly: plan + ı = planı.

mahvetti is the verb mahvetmek in the past tense, 3rd person singular.

Breakdown:

• mahvetmek = to ruin
• stem: mahvet-
• past suffix: -ti
• mahvetti = ruined

This is the common -DI past in Turkish, often used for a completed past event.

Also, the double tt is normal:

• stem ends in t
• suffix begins with t
• so you get mahvetti

Because the most neutral Turkish word order is Subject + Object + Verb.

So here:

• Yağmur = subject
• planı = object
• mahvetti = verb

That gives:

• Yağmur planı mahvetti

English usually prefers Subject + Verb + Object, but Turkish usually puts the verb last.

Turkish word order is flexible, though. Other orders are possible for emphasis, but verb-final is the default.

A big clue is that planı has the object marker -ı.

That tells you planı is the direct object, so Yağmur is the subject.

In other words:

• Yağmur = the thing/person doing it
• planı = the thing being affected
• mahvetti = the action

Turkish subjects are usually unmarked, while specific direct objects are often marked with the accusative.

It can be either.

• yağmur = rain
• Yağmur = also a common Turkish first name

Because it is at the beginning of the sentence, it is capitalized anyway, so capitalization alone does not tell you which meaning is intended.

So only context or the given translation tells you whether this means:

• Rain ruined the plan or
• Yağmur ruined the plan

Turkish does not have a separate definite article like English the.

Instead, definiteness is often shown by:

• context
• word order
• case marking, especially on direct objects

So planı often corresponds to the plan.

Compare:

• planı = the plan / that specific plan
• bir plan = a plan

So the idea of the is present, but Turkish expresses it differently.

ğ is called yumuşak ge, or soft g.

In modern Turkish, it usually is not pronounced like a hard English g. Instead, it often:

• lengthens the previous vowel, or
• creates a very slight glide

So Yağmur is roughly pronounced like yaa-mur or yah-moor, not yag-mur.

This letter often confuses English speakers because it usually changes the sound around it rather than being pronounced strongly itself.

You would normally use bir and remove the accusative ending from the object:

• Yağmur bir plan mahvetti

That means Yağmur / the rain ruined a plan.

So the contrast is:

• planı = the plan
• bir plan = a plan

This is one of the most important object patterns in Turkish:

• specific object → often takes accusative
• non-specific object → usually does not take accusative