Your question is a general question not specific to future tense.
Let´s put it this way:
When a word ends with k, in what cases do we need to apply k to ð mutation?
Answer:
When you add a suffix that starts with a vowel, k mutates to ð, othwersise it doesn´t change.
Examples:
Ayak = foot
Ayaðým = my foot (after suffix -ým which starts with vowel ý)
Balýk = fish
Balýða = to the fish (after suffix -a which starts with vowel a)
Yatak = bed
Yataða = to the bed (after suffix -a which starts with vowel a)
Yatakta = in the bed (after suffix -ta which starts with consonant t)
The rule has exceptions...
I used the word "safety" just because you used it...It is not safety (but insecure) especially when you join in and you join very often! 
