How can I search this PIE root '*dens-'

I’m studying the etymology of vocabulary to memorize vocabulary efficiently and be able to inference its composition. Take ‘autocracy’ as an example, it is composed of 2 words: ‘auto’ and ‘cracy’. I can inference its meaning of ‘government by single or small people and with unlimited power’.

But some words can not simply be composed by obvious words, like ‘didactic‘. I can only get its history and ‘PIE root’ information:

“fitted or intended for instruction; pertaining to instruction,”
1650s, from French didactique, from Latinized form of Greek didaktikos
“apt at teaching,” from didaktos “taught,” past participle of
didaskein “teach,” from PIE *dens- “to learn” (source also of Sanskrit
dasra- “effecting miracles”). Related: Didactical; didactically.

When I search for the PIE ‘dens’, I just can not find anything about this root.
And the second question is, what is the relationship between ‘dens’ and ‘didactic’?
I can hardly associate them to inference the meaning of ‘didactic’, although the root ‘dens’ and ‘didactic’ have similar ‘teach’ meanings.

So, is there any referable site that can get the meaning of PIE roots?
And, why the ‘dens’ have relationships with the ‘didactic’?