For more information on the negative particle לְבִלְתִּי see Gesenius §114s. Also, see BDB pg. 116b.
In the video on Ruth 1:13b we came across the verb תֵּעָגֵ֫נָה (Niph’al Imperfect 2fp עגן). In the video I briefly talked about what likely happened to the third root nun. Many of the grammars that are available to you do not address this word, with the exception of Joüon-Muraoka. Therefore, anything said here . . . just hold it tentatively.
In the video I said that the R3 nun, falling in the silent shewa position, likely assimilated into the nun of the 2fp ending נָה thereby lengthening the preceding vowel to sere. I likened the assimilation of the R3 nun of this verb to another R3 nun verb: קטן. The Qal Imperfect 2fp of קטן is תִּקְטַ֫נָּה. The R3 ן of קטן, falling in the silent shewa position, assimilated into the נ of נָה. See Gesenius §44o and Ezek. 17:23. But if the nun assimilated, what happened to the dagesh forte? All the requirements allowing the dagesh forte to be omitted are not met in our verb in Ruth 1:13b. For those requirements see Gesenius §20l-o and Joüon-Muraoka §18l-m.
Joüon-Muraoka, on the other hand, states that an R3 nun does not assimilate, stating that the author of Ruth (Samuel, in my opinion), writes תֵּעָגֵ֫נָה for תֵּעָגֵ֫נָּה (see §17g). It seems that Joüon-Muraoka is stating that instead of the R3 nun assimilating, only one nun was written instead of writing two nuns. Joüon-Muraoka does give other examples of this happening.
In the end, I’m not entirely sure what’s going on. I lean towards Joüon-Muraoka brief explanation since the requirements allowing for the omission of the dagesh forte are not met. If anyone else has an idea I’m all ears. However, since there are examples of other verbs with R3 nun doing funny things it shouldn’t cause too much consternation. Just take a look at all the funny stuff the verb נתן does.