As stated earlier, the Swedish sk-sound changes depending on the word. The best indicator for when to pronounce it as "sk", and when to pronounce it as the "sj" is to look at whether it's a vowel of consonant that follows, and what kind.
If it's an soft vowel, E, I, Y, Ä, Ö, that follows, it's "sj".
Skärp - Sjärp
Sked - Sjed
Skymning - Sjymning
If it's a hard vowel A, O, U, Å, or a consonants that follows, it's pronounced as it's spelled.
Skräp
Skapa
Skruv
I hope this helps!