1. Serviceably.

Nuttall's Standard dictionary of the English language By Nuttall, P.Austin

• usefull,
• useauly,
• useually,
• awefully,
• ussaully,
• useully,
• isfully,
• usuefull,
• usuaully,
• useaully,
• sussfully,
• sussefully,
• dasefully,
• usufull,
• useally,
• uselly,
• usaully,
• usefuly,
• userally,
• usfull

• useful,
• usefulness.

The adjective form of "usefully" is "useful".

The plural form of the word "usefully" is "usefully." The word remains the same in both singular and plural form.

The singular form of the word "usefully" is "useful"