And then when many aduersities and tribulations shall come vpon them, this song shall answere them to their face as a witnesse: for it shall not be forgotte out of the mouthes of their posteritie: for I knowe their imagination, which they goe about euen now, before I haue brought them into the lande which I sware.
Deuteronomy 31:19 Now therefore write ye this song for you, and teach it the children of Israel: put it in their mouthes, that this song may be my witnesse against the children of Israel.
Deuteronomy 31:20 For I will bring them into the land (which I sware vnto their fathers) that floweth with milke and honie, and they shall eate, and fil them selues, and waxe fat: then shall they turne vnto other gods, and serue them, and contemne me, and breake my couenant.
Deuteronomy 31:21 And then when many aduersities and tribulations shall come vpon them, this song shall answere them to their face as a witnesse: for it shall not be forgotte out of the mouthes of their posteritie: for I knowe their imagination, which they goe about euen now, before I haue brought them into the lande which I sware.
Deuteronomy 31:22 Moses therefore wrote this song the same day and taught it the children of Israel.
Deuteronomy 31:23 And God gaue Ioshua the sonne of Nun a charge, and said, Be strong, and of a good courage: for thou shalt bring the children of Israel into the lande, which I sware vnto them, and I will be with thee.
The verse centers on "aduersities", "tribulations", "shall", "come", "vpon", "song", and "answere". It is saying that the verse draws attention to "aduersities" and "tribulations", so its meaning should be read from those terms before moving to application.
The nearby context moves from verse 20's "For I will bring them into the..." into verse 22's "Moses therefore wrote this song the same...", so "aduersities" and "tribulations" belong inside that flow. In Deuteronomy context, the local focus is covenant, worship, and faithfulness.
A plain takeaway is to answer the verse's own emphasis on "aduersities" and "tribulations" with trust shaped by these words, not by a vague optimism outside the passage.