Outside: sigh, gasp 叹 (The mouth 口 should be (covered) with the hand 又, when: sighing.), inside: to accomplish (Past-suffix) 了 (The still arm-less 了 child 子 should be completed.), one 一
Mnemonic
He sighed at the completion because it was so urgent.
The app has many other functions, such as character selection according to difficulty, practice programs, handwriting recognition and success statistics!