Yehoshua 8:30- יבנה= built or will build?

Question

Yehoshua (8:30)

אָ֣ז יִבְנֶ֤ה יְהוֹשֻׁ֙עַ֙ מִזְבֵּ֔חַ לַֽיהוָ֖ה אֱלֹהֵ֣י יִשְׂרָאֵ֑ל בְּהַ֖ר עֵיבָֽל׃

At that time Joshua built an altar to the LORD, the God of Israel, on Mount Ebal, (Sefaria translation)

Grammatically, יבנה should mean “(he) will build” (future). But according to the P’shat of the passuk (and the translation), it means “he built” (past)!

Do any meforshim say why it’s written in past tense, but understood in future tense?

Alternatively, is there a grammar rule that I don’t know about that makes this make sense?

Answer 1

The Talmud (Sanhedrin 91b) proves "resurrection of the dead" from this seemingly grammatical error:

It has been taught: R. Meir said, Whence do we know resurrection from the Torah? From the verse, Then shall Moses and the children of Israel sing this song unto the Lord: not sang but shall sing is written: thus resurrection is taught in the Torah. Likewise thou readest, Then shall Joshua build an altar unto the Lord God of Israel: not 'built', but shall build is written: thus resurrection is intimated in the Torah.