Most people know that the Biblical meaning of a word is determined by its underlying three-letter root. The Biblical root can be conjugated in different a) persons, b) tenses, c) pluralities, d) genders, e) constructions and f) modalities. For example I watched Shamarti has a different conjugation then I will be watched EShaMer even though both phrases will use the same 3 letter Hebrew root.

Rashi will generally give rules of grammatical conjugation when the conjugation involves a rare form. Verse Gn47-19b has the word Tauv-Shin-Mem, TaySham which Rashi translates as become desolate; when a land lies fallow without being worked on it becomes desolate. Here Rashi views Taysham as the passive future form of the root Shin-Mem-Mem, Shamam which means to desolate. Shin-Mem-Mem is a double verb of the form, X-Y-Y and its conjugations are covered in table 10 of the appendices of the Ibn Shoshan dictionary. This table gives the form Tisham while the verse uses the actual form TaySham. Moshe Silverman's grammatical konkordance lists this verse in form #3444#13 and points out that Is51-06 gives the form Taychath for a future passive because the Cheth is a guttural letter. Moshe points out that The application of this form to the root Shin Mem Mem - Taysham/ Taychath - is peculiar since the shin is not a guttural letter.

To recap
• Future passive for a double root is typically Tisham
• Future passive for a double root with a middle guttural letter uses the form Taychath
• Future passive for the double root Shin-Mem-Mem uses the Taysham form when we would really expect Tisham.
• Since this form is unexpected Rashi comments on it.
• The entire verse would be translated as follows: Wherefore should we die before thine eyes, both we and our land? buy us and our land for bread, and we and our land will be bondmen unto Pharaoh; and give us seed, that we may live, and not die, and that the land not become desolate {from lack of ploughing].