string vs stringing

[Alice Nora]

English Language Tests, Intermediate level

ESL/EFL Test #267 "Irregular Verbs Test (15)", question 10

Vienna can't ......... her thoughts together in front of large groups because she gets really nervous.

(a) string
(b) strung
(c) stringing

English Language Tests, Intermediate level

ESL/EFL Test #267 "Irregular Verbs Test (15)", answer 10

Vienna can't string her thoughts together in front of large groups because she gets really nervous.

Correct answer: (a) string

Your answer was: incorrect
Vienna can't stringing her thoughts together in front of large groups because she gets really nervous.
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why not stringing?

Alice Nora

[Mister Micawber]

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-Ing form does not follow any auxiliary verbs other than 'be'.
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Native English teacher at Mister Micawber's