Mayıs planını değiştirmedim; yazlık kıyafetleri dolabın önüne koydum.

Usually no. Here Mayıs planı is a noun compound, so it means the plan for May or the May plan, not that May owns the plan.

This pattern is very common in Turkish:

• yaz tatili = summer holiday
• okul çantası = school bag
• Mayıs planı = May plan / plan for May

So Mayıs is identifying the type or time of the plan.

Because it is the direct object of değiştirmedim and it is specific. In Turkish, specific direct objects usually take the accusative ending.

So:

• Mayıs planı = May plan / the plan for May
• Mayıs planını = the May plan, as the thing being changed

The sentence is talking about one particular plan, not just any plan.

It is the result of two layers:

1. planı — the compound form in Mayıs planı
2. the accusative ending — marking it as a specific direct object

When a noun already has this kind of third-person/compound ending, the accusative appears as -nı / -ni / -nu / -nü.

So:

• Mayıs planı
• Mayıs planını

The n is a buffer consonant.

Because değiştirmek means to change something, while değişmek usually means to change in the sense of to become different.

Compare:

• Plan değişmedi. = The plan did not change.
• Planı değiştirmedim. = I did not change the plan.

Since the speaker is talking about changing the plan, Turkish uses the transitive verb değiştirmek.

It breaks down like this:

• değiştir- = change something
• -me- = not
• -di- = past tense
• -m = I

So değiştirmedim means I did not change.

Yazlık means for summer, summer-type, or suitable for summer.

So yazlık kıyafetler means summer clothes.

The ending -lık / -lik / -luk / -lük often makes words that mean meant for or related to something:

• kışlık mont = winter coat
• günlük kıyafet = everyday clothing
• yazlık kıyafetler = summer clothes

Because it is the specific direct object of koydum. Turkish usually marks a specific direct object with the accusative.

So:

• kıyafetler = clothes
• kıyafetleri = the clothes

Here it means the summer clothes as a definite set of clothes.

Yes, by itself kıyafetleri can be ambiguous.

Depending on context, it can mean:

• the clothes
  or
• his/her clothes

In this sentence, it is understood as the clothes because it is the direct object of koydum and matches the overall meaning.

This is a common sound change in Turkish called consonant softening.

When some nouns ending in p, ç, t, k take a vowel-initial suffix, the final consonant often changes:

• p → b
• ç → c
• t → d
• k → ğ / g

So:

• dolap = closet, wardrobe
• dolabın = of the closet / the closet's

It is dolap + ın, but the p softens to b.

This is a very common Turkish structure.

• dolabın = of the closet
• önü = its front / the front of it
• önüne = to its front / to the area in front of it

So dolabın önüne means to in front of the closet, which English naturally expresses as in front of the closet after a verb like put.

Turkish often builds location expressions this way:

• masanın üstüne = onto the table / to the top of the table
• kapının yanına = next to the door
• evin arkasına = behind the house

Because koydum expresses movement or placement toward a location.

Turkish uses:

• -e / -a for motion toward a place
• -de / -da for being at a place

So:

• dolabın önüne koydum = I put it in front of the closet
• dolabın önünde = in front of the closet, located there

If the sentence were describing where the clothes already are, önünde would make more sense.

Because Turkish often leaves out subject pronouns when the verb already shows the person.

Both verbs here are first-person singular:

• değiştirmedim = I did not change
• koydum = I put

So ben is understood automatically. It would only be added for emphasis.

Not completely. Turkish word order is flexible, but the neutral pattern usually puts the verb at the end of the clause.

Here the clauses are:

• Mayıs planını değiştirmedim
• yazlık kıyafetleri dolabın önüne koydum

This is a very natural order in Turkish. Other orders are possible for emphasis, but final-position verbs are very common.

The semicolon links two closely related statements:

• Mayıs planını değiştirmedim
• yazlık kıyafetleri dolabın önüne koydum

It shows a stronger break than a comma, but it still keeps the two ideas connected. A period could also be used, but the semicolon makes them feel like part of the same thought.