Learn Icelandic: Af hverju lest þú ekki leiðbeiningarnar sjálfur? - Why do you not read the instructions yourself?

Sjálfur is an intensifying pronoun here emphasizing the subject þú (you), not the instructions. Because þú is the subject of the sentence, it is in the nominative case, so sjálfur must match it. It is often placed at the end of the clause for natural emphasis.

Why use the masculine form sjálfur here? What if I am talking to a woman?

Sjálfur always agrees with the gender of the person being addressed. This sentence assumes the listener is male. If you were speaking to a woman, you would use the feminine nominative form sjálf, making it: Af hverju lest þú ekki leiðbeiningarnar sjálf?

What case is leiðbeiningarnar, and what is its root word?

It is in the accusative case because it is the direct object of the verb lesa (to read). The root word is leiðbeining (instruction), which is a feminine noun. Here it is in the plural definite form (the instructions), which adds the -nar ending.

Why does the verb lest come before the subject þú?

Icelandic strictly follows the "V2" (verb-second) rule. Because the question phrase Af hverju (why) takes the first position in the sentence, the conjugated verb lest must take the second position, pushing the subject þú to the third spot.