Learn Turkish: Yürüyüş için ormana gidecektim. - I was going to go to the forest for a walk.

This combines the future suffix -ecek with the past tense suffix -ti. It breaks down as: gid- (go) + -ecek (future) + -ti (past) + -m (I). Putting the past right after the future creates the meaning "I was going to [do something]" (but usually implies you didn't end up doing it!).

The verb for "to go" is gitmek. Why is it spelled with a "d" in gidecektim?

This is due to consonant softening (mutation). When the verb root git- gets a suffix starting with a vowel, like the future suffix -ecek, the hard t softens to a d. This makes it flow better when speaking!

Why is the word order yürüyüş için instead of "için yürüyüş" for "for a walk"?

In Turkish, words like "for" (için) are postpositions. Unlike English prepositions which come before the noun, Turkish postpositions always come after the noun they relate to. So, "for a walk" translates literally as "walk for" (yürüyüş için).

Why is there an -a at the end of ormana?

Orman means "forest". The verb gitmek (to go) requires you to show the direction of your movement using the dative case (meaning "to" or "towards"). The -a is that "to" suffix, making ormana mean "to the forest".