Learn Arabic (Modern Standard): كتب الجد وصية واضحة.

In Arabic, the most standard sentence structure is Verb-Subject-Object (VSO). So, the verb كتب (wrote) naturally comes before the subject doing the action, الجد (the grandfather).

Why do both وصية and واضحة end with the letter ة (taa marbuta)?

The noun وصية (a will/testament) is grammatically feminine. In Arabic, adjectives must match the gender of the noun they describe. Therefore, the adjective meaning "clear" adds the feminine ending ة to become واضحة.

Why is there no ال (al-) on the words for "clear will"?

Adjectives must match their noun in definiteness. Because he wrote "a" clear will (indefinite) rather than "the" clear will, the noun وصية does not take the definite article ال. Consequently, the adjective واضحة must also be indefinite and drops the ال.

This lesson is about fractions. How would I change this sentence to say "The grandfather wrote a third of a will"?

You would use the fraction ثلث (a third) and say كتب الجد ثلث وصية. If you wanted to say "a third of THE will", you would add ال to make it كتب الجد ثلث الوصية.