Learn Icelandic: Þau vilja alla sósuna.

alla is the feminine accusative singular form of the pronoun allur (all). It must match the gender, number, and case of the noun it describes. Since sósuna (the sauce) is feminine, singular, and in the accusative case, the word for 'all' becomes alla.

Why are alla and sósuna in the accusative case?

The verb vilja (to want) takes a direct object in the accusative case. Since the sauce is the thing being wanted, both the noun (sósuna) and the pronoun attached to it (alla) must be in the accusative case.

How exactly is the word sósuna formed?

It is a combination of the noun sósa (sauce) and the definite article suffix ('the'). Because the verb makes it accusative, the base word sósa changes to sósu. Then, the feminine accusative suffix for 'the' (-na) is added to the end, giving us sósuna.

Why use þau for 'they' instead of þeir or þær?

Þau is the neuter plural form of 'they'. In Icelandic, you use it when referring to a mixed group of people (such as men and women together) or a group of neuter nouns. If the group was only male, you would use þeir, and if only female, þær.