stammer|stammered|stammering

他紧张得说不出话来，只能结结巴巴地道歉。

He was so nervous that he could only ___stammer an apology.

她结结巴巴地说：'我……我的书包忘带了。'

She ___stammered, 'M-m-my bag is left at home.'

他在演讲时因为紧张，结巴得更厉害了。

His ___stammering became worse during the presentation due to nervousness.