Learn Icelandic: Pabbi hefur ekki séð þig lengi. - Dad has not seen you for a long time.

In the Icelandic present perfect tense, the negation ekki (not) usually goes right after the conjugated helper verb (hefur) and before the main action verb (séð). This is similar to English where "not" splits the two verbs in "has not seen".

Why is the pronoun þig used here instead of þú?

Þig is the accusative case of þú (you). The verb sjá (to see) forces its object into the accusative case because "you" are the one being seen. This rule applies even when the verb is in the present perfect form (hefur séð).

What form of the verb "to see" is séð?

It is the past participle (often called the supine form in Icelandic) of the verb sjá. Whenever you use the helper verb hafa (to have) to say someone "has done" something, the main verb must be in this supine form. For sjá, that form is always séð.

Why is the time adverb lengi at the very end of the sentence?

Adverbs of time, like lengi (for a long time), typically go at the end of the sentence in Icelandic. You first build the core sentence (Subject + Helper Verb + ekki + Main Verb + Object), and then add the time adverb last, just like in English.