Rashi: It is past tense, for the accent is under the Shin (penultimate syllable). "Uva'Boker Hi Shavah" (Esther 2:14) is present tense; the accent is under the Beis (last syllable). The same applies to all such verbs [in which the last letter of the root is Hei].

Vilna Gaon: Your Yevamah wanted to cling to me, but she left. Presumably, also your intent to cling to me is not complete; in the end, also you will leave me. If so, go now - why separate from your Yevamah?

Malbim: Since she returned to her nation, she returned to her gods (Midrash). She did not want to enter the creed of Yisrael. Presumably, also your intent is so - if so, return like her!

Tzitz Eliezer (17:42, citing Nachalas Binyamin): If one converted without a Shevu'ah, and later reverted to his old ways, he is not like a Yisrael Mumar. He may revert ¹ !

Tzitz Eliezer (17:42, citing Ohel David citing Tzafnas Pane'ach): Rus accepted all the Mitzvos, but did not immerse. If she will immerse to convert, it works retroactively from when she accepted Mitzvos ² . (If not, she never converted.)

The Kusim were assumed to be proper converts. When Chachamim found that Kusim were still serving their idols, this revealed that their conversion was never valid (Tosfos ha'Rosh Chulin 6a). Perhaps Na'ami was testing Rus and Orpah - if they will return to their nation, this shows that their conversion was never valid. According to the Zohar Chadash, that their father forced them to convert, this is very similar to the Kusim, who converted due to fear of lions! (PF)