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We propose a Treasury Halpha (F657N) imaging survey that will measure the sizes (radii) and structures (e.g. shell fraction) for >50 comma 000 nebulae comma including HII regions comma supernovae comma and planetary nebulae across the full range of galactic environments in 19 nearby main-sequence galaxies. These targets form a representative sample of massive comma star-forming galaxies comma and these proposed observations comma only possible with the HST comma will complete a rich dataset of i) VLT-MUSE IFU comma providing spectroscopic information for all identified nebulae ii) multi-filter broad-band HST imaging of >40 comma 000 clusters comma iii) ALMA spectral line mapping of >20 comma 000 molecular clouds and iv) >100 hours of upcoming near- and mid-IR imaging from JWST probing dust emission and embedded star formation. Together these maps and higher-level data products are crucial for answeringdoublePoint (1) What is the dominant mechanism for pre-SNe stellar feedbackquestionMark (2) On what timescales do young clusters clear their birth-cloudsquestionMark (3) What processes are responsible for cloud destructionquestionMark (4) What role does escaping radiation play in larger-scale galactic evolutionquestionMark Taken together comma the products obtained as part of this Treasury program will provide the definitive high resolution view of HII regions in nearby galaxies comma essential to answer questions fundamental to our understanding of star formation and stellar feedback. These observations are needed now to form a crucial part of Hubble.s lasting legacy.