Why is it called okuldaydim? Why is it not called okuldadim? I assume so far, that when you make past tense out of a verb, you put in a "y" before the "di", but im not sure. Can anyone please explain this to me?

Thank you in advance

In this case it´s past tense of ´to be´ rather than making past tense out of a verb. Often you need to add a ´y´ after a vowel, before adding a suffix. For past tense with a verb, you don´t need to add a ´y´.

This link has everything for ´to be´:

http://www.turkishlanguage.co.uk/tobepos.htm

.

About the use of the buffer ´y´:

http://www.turkishclass.com/forumTitle_61671

1. past to be + adjective: yaşlıydı, he was old

2. past to be + noun: siyah bir arabaydı, it was a black car

3. And furthermore all past* suffixed verbs ending with a consonant. 

- past conditional: olsaydı, if it was

- past necessitive: aramalıydım, I had to call

- past subjunctive: tıklayaydım

*Past being either -di or -miş

Follwing that rule: başla+mış+ım is without y because the verb stem is not suffixed with -sa or -malı or -yayım or such.

And it is başla+sa+y+mış+ım because başla is suffixed with the conditional suffix -sa.