Questions & Answers about Если я соскучусь по другу, я напишу ему сообщение в чате.
Russian commonly uses the future in both parts when you mean a real future situation:
- Если я соскучусь… = If I end up missing… / If I start to miss… (future)
- я напишу… = I will write… (future)
English often avoids will in the if-clause, but Russian does not have that restriction.
- скучаю (imperfective, present) = I miss (him) / I’m missing (him) (ongoing feeling now).
- соскучусь (perfective, future) = I will come to miss (him) / I’ll start to miss (him) (the moment the feeling appears, or the fact that it will occur at some point).
So the sentence implies: at some point later, you’ll feel that missing, and then you’ll write.
With скучать / соскучиться in the meaning to miss someone, Russian uses по + dative:
- скучать по кому? → по другу (dative singular of друг)
Using друга (genitive/accusative) would be ungrammatical with this verb in this meaning.