I am having difficulty with describing things in the various tenses in the nominative case (the subject of the sentence), particularly with the verb "to be".

Я играю в теннис с молодым энергичным братом в мирном парке

but

How do you say -

"He thinks it is interesting"

"I think this russian book is interesting"?

Он дамает, что интересно?
Я дамаюб что яусская книга интересно?


If I am by coincidence right, then, how would I say "this book was interesting". "This book will be interesting"?

Большое спасибо за помощь

Джек