"He helped me with my work yesterday."
Translation:Amenisaidia na kazi yangu jana.
My guess is it's because the perfect tense can be used here in Swahili to emphasise that the work is currently complete, and we simply can't do this in English because the word "yesterday" rules that out. The English present perfect tense is incompatible with time adverbs indicating finished time-spans (such as "yesterday"), but as far as I know, the Swahili perfect has no such restriction. Like, you can't say "He has helped me yesterday" but it appears you can say "Amenisaidia ... jana".
That being said, it should definitely also accept "Alinisaidia".