diyorsan is like if clause on present continious tense i mean its like if you are saying ... in english, but dersen is like if clause on simple present tense i mean if you say ... in english. actually this both words meaning is very similar to each other. i hope you understand ^^.

Sandim is like did in english but saniyordum is like was in english.You can use both of them actually no big different.

so, the difference is "past simple" and "past continuous"

sanmak: to suppose, to think san + dı + m >> sandım san + ıyor + du + m >> sanıyordum Odanda olmadığını sandım. (I thought you were not in your room.) Odanda olmadığını sanıyordum. (I thought you were not in your room.) ((i had been thinking you were not in your room (until i noticed)) .... yazmak : to write yaz + dı + m >>yazdım yaz + ıyor + du + m >>yazıyordum Beni aradığında hocama e-posta yazıyordum. (I was writing an e-mail to my teacher when you called me.) İki önemli soru yazdım. Ne zaman cevaplar, bilmiyorum. (I wrote two important questions. I don't know when he'll reply me.)

(i can't colour the words here. if you asked your question in the "discussion" or "notebook" sections, it would be possible to colour. so my explanations here may seem complicated) the personal endings of 1st type: -im -sin -* -iz -siniz -ler (*-dir has four or five other tasks. in many sources it's taught equivalent to "it is" but i will explain it to you later) the personal endings of 2nd type: -m -n - -k -niz -ler/- ... -di: simple past -di + m........-dim.....i -ed -di + n.........-din......you -ed -di + -..........-di.........he she it -ed -di + k.........-dik.......we -ed -di + niz......-diniz....you -ed -di + ler........-diler....they -ed ...................................................................... imek: am is are (an archaic verb) i- i + di + m....>>idim...: i was i + di + n.....>>idin....: you were i + di + .......>>idi......: he she it was i + di + k.....>>idik.....: we were i + di + niz..>>idiniz..: you were i + di + ler...>>idiler....: they were -yor: present continuous -yor + um.....>>-yorum....: i'm ...ing -yor + sun.....>>-yorsun....: you are ...ing -yor + ...........>>-yor...........: he she it is ...ing -yor + uz.......>>-yoruz.......: we are ...ing -yor + sunuz.>>-yorsunuz.: you are ...ing -yor + lar.......>>-yorlar........: they are ...ing -yor + idi + m....>> -yordum...: i was ... ing -yor + idi + n.....>> -yordun....: you were ...ing -yor + idi + .......>> -yordu.......: he she it was ...ing -yor + idi + k.....>> -yorduk.....: we were ...ing -yor + idi + niz..>> -yordunuz..: you were ...ing -yor + lar + dı...>> -yorlardı......: they were ...ing