Learn Arabic (Modern Standard): اخترعت المهندسات أول حاسوب في الأربعينيات. - The female engineers invented the first computer in the forties.

To form a decade, you start with the oblique form of the tens number, which is أربعين (forty). Then, you attach the Nisba suffix ـيـ (-iyy-) to turn it into a relational adjective, and finally add the feminine sound plural ending ـات (-āt) to group those years together. Adding the definite article الـ gives you الأربعينيات.

Since الأربعينيات follows the preposition في, would the base change to الأربعونيات if it were the subject of the sentence?

No, it wouldn't. When naming decades, the oblique form of the cardinal ten (ending in ـين) is frozen as the base word before adding the suffixes. You will always use الأربعينيات for 'the forties', regardless of its grammatical case in the sentence.

Why is the verb اخترعت (invented) in the feminine singular when the subject المهندسات (female engineers) is plural?

This is a classic rule of Verb-Subject-Object (VSO) sentences. When the verb comes before an explicitly stated human subject, it remains in the singular form and only matches the subject in gender. Because المهندسات is feminine, we use the feminine singular اخترعت.

Why doesn't أول (first) match حاسوب (computer) in definiteness like a regular adjective?