knight v Bishop

Knights and bishops are equal, but depending on the specific position certain pieces are stronger or weaker. Also the bishop pair is a small plus, so you can think of the first bishop you trade as being a bigger loss than the 2nd.

But this is all relative. Consider, for example, refining a move order so that instead of using 4 moves to achieve a setup, you use 3. These are very small pluses we're talking about, and probably not noticeable for most players. Even when 2000 rated player say something about the bishop pair I tend to laugh to myself. It usually takes a lot of good play, dozens of moves, including a well played endgame to prove such an advantage.

Anyway, a few common tips:

1) Usually players don't trade their first bishop for a knight unless it damages the pawn structure. For example the Ruy Lopez exchange variation which doubles black's pawns. There are exceptions, but this is common enough to be worth mentioning IMO.

2) Knights, as the only piece than can hop over others, are relatively stronger in closed positions, and in general are stronger with many pawns still on the board. Bishops, especially the bishop pair, are best on an open board with few pawns (which is why they can really shine in an endgame).

3) If your central pawns are locked on light squares, then your light square bishop is your "bad" bishop. This just means since friendly pawns obstruct it, it tends to not be as mobile as the dark square bishop which in this case would be your "good" bishop.

In general, you shouldn't rush to trade off the good bishop, as in addition to being more mobile on average, it also complements the pawns (it covers the color of square they don't).

So opening moves like

1. d4 d5 2. e3 e6 3. Nf3 Nf6 4. c3 Nc6 5. Bb5

Already a silly move. This is white's "good" bishop and white is acting like he will trade it.

5...Bd7 6. O-O a6 7. Bxc6

Another bad move. White trades off his good bishop, and also loses the bishop pair without damaging black's pawn structure.

An exception (there are always exceptions to rules like these) would be, for example, if the e5 or d4 square were for some reason very important. Then white may go ahead and trade his "good" bishop for the c6 knight, just to gain more control of one of those squares.

As a rule of thumb, you can think of bishops at 3.5 points vs knight being 3.  However, the board position is important too as mentioned above.  One thing that's not mentioned too often is that beginners usually find bishops easier to use (so you may be able to more effectively use a bishop, e.g. helping in checkmates, forks, or other tactics)... this can be a double edged sword though: your opponents (being another beginner) can defend against a bishop easier than knight.

I'd say, keeping bishops instead of knights should help you at your stage.

As mentioned earlier, the Bishop Pair, in open position endgames are usually stronger.

Bishop against Knight in the end game depends on the pawns but usually favors the Knight.

Bishops of opposite colors are often/usually drawn even when one side has a 1 or 2 pawn advantage.

Bishops + Rook pawn which Queens on square not of the Bishops color is drawn.

King and 2 Bishops can win against lone King.

King and 2 Knights is a draw against a lone King.

These are some things to consider when the Bishop/Knight trade is possible.

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In chess when you acquire experience you learn about a concept called the MINOR EXCHANGE.  This when you exchange a B for a N.  This is because the Bishop is considdered to be the superior piee in most positions, especially in the endgame. The Bishop can get completely across the board in one move.  It takes the Knight 4-5 moves to get completely across the board.  The Bishop can gain or lose a tempo.  The knight cannot gain or lose a tempo.  There are many other factors that you learn.  In blocked pawn positions thee Knights are superior to the Bishop.  Bishops can trap a Knight on the edge of the board.  There are many other factors.   But by and large thee B is superior to the N