I a trying to earn how to spell and construct Turkish words. Could someone please explain to me why 25 dollars a meter is "Metresi 25 dolar". I do not understand the meaning of the si at the end of metre. Another one is why is Four times a month "Ayda dört kere". Again I don´t understand the "da" at the and of ay.  One more before I go, why is there an i at the end of her şey in the sentence "Her şeyi daha temiz yapti."?   Thank you so much! I am sure I can bring you many more questions later. 

I am only a learner as well, but here is my attempt to explain.

Whenever you see a "si" suffix on the end of a noun it means it "belongs to" or is "owned" by the previous noun.

Onun kedisi = his/her cat

Ali´nin arabası = Ali´s car

There is a more extensive explanation when you click on here

But now you are thinking there is no noun preceding "metresi". We can assume that the missing noun is ´onun´ (his/her/its) . The material´s (its) metres (cost) is 25 dollars. 

The da/de/ta/te suffix means in/on/at.

Ay = month/moon

Ayda = in the month ( and also maybe on the moon!)

Ayda dört defa = Four times in the month.

See here and there for more details on static or locative suffixes

The ´i´ suffix at the end of her şey indicates that it is the object of the final verb.

It makes the noun a ´definite´ object of the verb.

Her şeyi daha temiz yaptı. He/she/it made everything cleaner (more clean)

Seni seviyorum. I love you.

Kitabı okuyorum. I am reading the book. (A specific book)

Kitap okuyorum. I am reading a book. (Any book, a book)

I am also sure you will have more questions now  

If my words look like rude, sorry but your questions are like "why English sentences construct by Subject Verb Object(SVO) order and not in another order" etc. It is a rule, there is no reason about it.

Henry´s explanation is just great. 

 Metresi 25 dolar.

-si is the accusative (direct object) form. (The ´s´ is a buffer letter here.) Literally it means something like "the meter belongs to/is possessed by/IS THE DIRECT OBJECT OF 25 dollars" 

 Ayda dört kere.

-da here means "in": Four times in a month.

    saat yedide = at seven o´clock

    burada = (in) here

    fabrikada = in the factory.

    evde = in the house, at home

Her şeyi daha temiz yapti. = He/she made everything cleaner (more clean).

 her şey = everything (nominative) - the form used if "everything" is the subject

 her şey-i = everything (accusative form) - the form used when "everything" is the

                 direct object, as in our example.

 Eg. telefon-u kulanıyor = he is using the telephone (direct object = accusative form)