[Mesorah] Va'avadum v'eenu osam

[Akiva Miller via Mesorah]

I'm having some trouble translating some words from the Bris Bein
Habesarim, Berishis 15:13. "Va'avadum v'eenu osam arba me'os shana."
My weak knowledge of Hebrew grammar is putting up some large
roadblocks, and I hope someone can help me out.

"Avadum" clearly means "they (Yisrael) will serve them (Mitzrayim)".
It cannot mean "they (Mitzrayim) will enslave them (Yisrael)," nor can
it mean "they (Yisrael) will be enslaved by them (Mitzrayim)". I reach
this conclusion from the use of "avadum" in Dvarim 31:20 and Yirmiya
8:2.

But I'm very fuzzy on what "eenu" means in this pasuk. Perhaps it
means "they (Mitzrayim) will torture them (Yisrael)", as in Shoftim
20:5. But then the words in Bereshis would have to switch
subject/object in the middle of the pasuk: "They (Yisrael) will serve
them (Mitzrayim), and they (Mitzrayim) will torture them (Yisrael)." I
suppose this is possible, but it is not a normal way of speaking. I
suspect that "eenu" actually means that "they (Yisrael) will BE
tortured BY them (Mitzrayim)", similar to Eicha 5:11. If so, then
Bereshis can keep the same subject/orbject for both verbs: "They
(Yisrael) will serve them (Mitzrayim), and they (Yisrael) will by
tortured by them (Mitzrayim)."

I note that the first verb here ("avadum") has the object appended to
it, while the object is a standalone word for the second verb ("eenu
osam"). That might be significant, but I don't know enough grammar to
see any difference in meaning, only a difference in style or spelling.
However, Onkelos may shed light on this point, because he makes the
object a standalone word in both cases, and uses different words for
those objects: "v'yifl'chun b'hon, veeanun yas'hon". I know enough
Aramaic to equate peh-lamed-ches with ayin-beis-daled, and the second
verb seems to have the same root in both languages. It is the object
here that attracts my attention: What is the difference between
"b'hon" and "yas'hon"?

All thoughts and ideas greatly appeciated.

Akiva Miller