48 Heb “caused me to know that I might know.” Many English versions supply an unstated object “their plots” which is referred to later in the context (cf. v. 19). The presupposition of this kind of absolute ellipsis is difficult to justify and would create the need for understanding an ellipsis of “it” also after “I knew.” It is better to see a bipolar use of the verb “know” here. For the second use of the verb “know” meaning “have understanding” see 394 ָידַע Qal.5.
49 Heb “Then you showed me their deeds.” This is another example of the rapid shift in person which is common in Jeremiah. As elsewhere, it has been resolved for the sake of avoiding confusion for the English reader by leveling the referent to the same person throughout. The text again involves an apostrophe, talking about the Lord to addressing him.

In other versions

The Lord made it known to me and I knew;

then you showed me their deeds.

Because the Lord revealed their plot to me, I knew it, for at that time he showed me what they were doing.

The Lord informed me, so I knew.

Then you helped me to see their deeds,

Then the Lord told me about the plots my enemies were making against me.

Moreover, Yahweh let it be made known to me, and I knew.

Then you showed me their deeds.

And the Lord hath given me knowledge of it, and I know it: then thou shewedst me their doings.

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