-It is להושיע.
-The imperative form is הושע hosha'.
-It would help if you were to include the exact hebrew spelling along with your transcriptions,so that I won't be left to guess which word you were referring to and whether you read it correctly.
In this particular case I can think of several options:
a.הושיעה-נא hoshia'-na,which I believe I have already explained - hoshia' being the long imperative form and 'na' used as a particle to express politeness or pleading.
b.הושיענה hoshie'na or hoshia'na,which better fits your transcription,but is less likely that you have encountered,and means 'save her'/'saved her'.
c.other possibilities are הושע-נא and הושענא,with the former similar in meaning to A above,and the latter being a holiday's name derived thereof.
HoneyBuzzard wrote:When you say "if," do you mean "because"? It was my impression that the vocal shewa under a daghesh forte was purely euphonic.
I do.
In its most basic definition,a vocal shewa is a vowel that undergone shortening due to the distancing of the stress in the word.