Oxford documents “happen to be/do” for chance occurrence. The ordinary event sense of “happen” is structurally different, so bare “happen doing” should not be treated as an interchangeable gerund option.
ReviewedEvidence2 sourcesSectionEnglish Usage
Quick answer
Use “happen to do” when an action occurs by chance. Bare “happen doing” is not the corresponding complement pattern.
Key details
Confusion Familygrammar-and-usage distinction
Rule Familyverb complement pattern
TermsHappen Doing, Happen To Do
Further guidance
Chance Pattern
Use happen + to-infinitive when the subject does or is something by chance.
Decision
For the chance construction, choose “happen to do”; do not replace the infinitive with a bare gerund.
Event Boundary
The ordinary event sense of “happen” does not create a parallel happen + -ing complement.
Sources and evidence
Sources are shown with the role they play in this guide. Historical or style-sensitive claims are kept within the evidence boundary described above.
Cambridge documents both admit + -ing and admit to + -ing for acknowledging something. Other passive patterns such as “be admitted to be” are separate constructions.
Both patterns are standard. “Advise waiting” recommends an action generally or in context, while “advise him to wait” names the person who is advised to perform it. A that-clause is another standard option.
When “afford” is followed directly by another verb, standard learner references use a to-infinitive: “afford to do.” An -ing form can appear elsewhere in a sentence, but it is not a like-for-like direct complement after “afford.”
“Agree to do” expresses consent to take an action. An -ing form normally follows a preposition in patterns such as “agree with doing,” which expresses approval rather than the direct promise-to-act construction.
Cambridge documents aim + to-infinitive for intention and aim at for directing effort toward an objective. Do not omit the preposition and present bare “aim doing” as equivalent to “aim to do.”
“Allow” can take an activity as its object, as in “allow smoking,” or a person/thing followed by a to-infinitive, as in “allow visitors to enter.” The patterns answer different structural questions: what is allowed versus who is allowed to do what.
