It's easy.

past tense suffixes are as follows:

-dı
-di
-du
-dü

-tı
-ti
-tu
-tü

1. After a vowel always use the "-d" version.
2. After an unvoiced consonant use the "-d" version.
3. Only after a voiced consonant use the "-t" version.
Voiced consonants are : p,ç,t,k,h,s,ş,f (hint "Fıstıkçı Şahap")

Ok, let's summarize 1,2 and 3:

If the last letter before the past tense suffix is "abcdegğıijlmnoöruüvyz" use "-d" version and if it is "pçtkhsşf" use "-t" version.

Remember it is only the last letter that counts for consonant harmony.
gitmek : to go
"git + ti + m" : I went
"git + me + di + m" : I didn't go

On affirmative last letter was voiced consonant, on negative last lette changed.

Now you know when to use t or d versions. But there are four of each. Which one do we use, say for d version?

-dı -di -du -dü

This time after consonant harmony it is "vowel harmony" that takes the stage.

In general :
a is followed by a or ı
e is followed by e or i
ı is followed by a or ı
i is followed by e or i
o is followed by a or u
ö is followed by e or ü
u is followed by a or u
ü is followed by e or ü

If we apply these to past tense:

if the last vowel is a use -dı
if the last vowel is ı use -dı

if the last vowel is e use -di
if the last vowel is i use -di

if the last vowel is o use -du
if the last vowel is u use -du

if the last vowel is ö use -dü
if the last vowel is ü use -dü

Example:
"gitmek"
verb stem is "git"
now you check two things:

1. is the last letter a voiced consonant?
2. what is the last vowel?

Yes, the last letter a voiced consonant and therefore I should use the t version.

The last vowel i and I should use "ti"

So we get "gitti".

example:
okumak > oku

1. is the last letter a voiced consonant? no
2. what is the last vowel? u
>>okudu

Notice that the same vowel and consonant harmony rules above apply to nouns and adjectives in past tense as well. Yes we use past tense suffixes with nouns and adjectives too.

In Turkish we don't have the "to be" (am, is, are, was, were, has been, had been).
So we say:

Hava güzel. > The weather is nice.
Hava güzeldi. > The weather was nice.

Very nice explanations. even i am impressed...
Thank you very much.

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