Learn Icelandic: Þernan sér þetta óhreina lak. - The maid sees this dirty sheet.

You would introduce a value judgment phrase like Það er hræðilegt að (It is terrible that) and change the verb to the present subjunctive. The present subjunctive of sjá for the third-person singular is sjái. The sentence would then become: Það er hræðilegt að þernan sjái þetta óhreina lak.

Why does the adjective take the form óhreina instead of the strong neuter form óhreint?

Adjectives must use the weak declension when they directly follow a demonstrative pronoun like þetta, a free definite article, or a possessive pronoun. While the strong accusative neuter of óhreinn is óhreint, the weak declension form for the neuter singular is óhreina.

If þernan (the maid) were the object being seen instead of the subject, how would the word change?

Þerna is a weak feminine noun, which means it ends in -a in the nominative singular but changes to -u in the accusative, dative, and genitive cases. If she were the direct object, you would need the accusative definite form, which is þernuna.

How do we know þetta óhreina lak is in the accusative case if the words look exactly the same as they do in the nominative?