Learn Turkish: Sen haklıydın, otobüs doluydu. - You were right, the bus was full.

Both haklı (right) and dolu (full) end in a vowel. When we attach the past tense suffix to a noun or adjective ending in a vowel, we must add the buffer letter -y- so two vowels don't crash into each other.

Why does the past tense suffix look different (-dı vs -du)?

That is 4-way vowel harmony at work! The past tense suffix changes to match the last vowel of the word. Because haklı ends in an undotted ı, it takes -dı. Because dolu ends in u, it takes -du.

How do we know haklıydın means 'you were right' and not 'I was right'?

The -n at the very end is the personal suffix for 'you' (singular). The breakdown is haklı (right) + -y- (buffer) + -dı (past tense) + -n (you). If it were 'I was right', it would be haklıydım with an -m.

Why doesn't doluydu have an extra letter at the end like haklıydın does?

otobüs (the bus) is an 'it' (third-person singular). In Turkish, the 'he/she/it' form doesn't take a personal suffix at the end. Just the word plus the past tense ending is enough to mean 'it was full'.