Learn Swedish: Min nya förare vinner loppet idag. - My new driver wins the race today.

Because it comes after the possessive pronoun min. In Swedish, whenever an adjective follows a possessive word (like min, din, or hans), it must take the definite form, which almost always ends in -a regardless of the noun's gender.

Why does the Swedish sentence use the present tense vinner if the race hasn't happened yet today?

Swedish frequently uses the present tense to talk about future events. Because the time marker idag (today) provides the context that this is happening later, vinner naturally translates to 'wins' or 'will win' in English.

How could I add 'as fast as possible' to this sentence using the current lesson's grammar?

You would use the adverb snabbt (fast/quickly) and wrap it in the construction: så snabbt som möjligt. The full sentence would be: Min nya förare vinner loppet så snabbt som möjligt idag.

What is the base form of loppet?

The base form is ett lopp (a race). Since it is an 'ett' word ending in a consonant, it takes the suffix -et to become the definite form loppet (the race).