Learn Turkish: Avukat adaletsiz bir dava açmamalıydı. - The lawyer should not have filed an unfair lawsuit.

It combines the verb stem aç (to open), the negative suffix -ma, the obligation suffix -malı (should), the buffer letter -y-, and the past tense suffix -dı. Together, -mamalıydı expresses an unfulfilled past obligation, meaning 'should not have done it' (implying the lawyer actually did it).

Why is the verb açmak (to open) used here for filing a lawsuit?

In Turkish, the fixed expression for 'to file a lawsuit' or 'to sue' is dava açmak (literally, to open a lawsuit). You cannot translate 'to file' literally here.

Why is the word order adaletsiz bir dava instead of bir adaletsiz dava?

When describing a noun with both an adjective and the indefinite article bir, the adjective almost always comes first in Turkish. The standard pattern is Adjective + bir + Noun.

How is the adjective adaletsiz formed?

It is made from the noun adalet (justice) and the suffix -siz (without or -less). It literally means 'without justice' or 'unfair'.