Don't understand "typical" ...c5 moves (or c4 for white)

See diagram: black's last move in both cases was ...c5. I rarely have the insight when to properly play c4 or ...c5. Can anyone help me how to think? What does black tangibly achieve in these positions right now?

I know in many openings, playing c4 or ...c5 helps with pressurizing the center, but too often I do not see how these kind of "typical" ideas help me. It's a structural problem that I have and I hope somebody can provide and aid in how to think when it comes to this. Not even sure what question exactly to ask.

Left: Black challenges white’s center. ...c5 has to be carefully timed with DSB on e7, because d4-d5 usually gives white a big space advantage. Tactical resources are vital in such positions.

In the diagram above, white is better after 8... c5 9. exf5 exf5 (9... cxd4 10. Nxd4 gxf5 11. Qe2 Qd7 12. Bh6 Rf7 13. h3 Nc6 14. Nxc6 bxc6 15. g4 black’s kingside is shaky) 10. d5, when black has no visible counterplay and a dangerous hole on e6.

8... fxe4 is better here. In the classical Dutch structure, ...fxe4 followed with ...Nc6 and ...d5 is a well-known plan to put pressure on white’s center, which equalizes comfortably in the diagram above.

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Right: Black fixes the c-pawns, which in itself is a common way to exploit doubled pawns. The timing of this move is questionable, though. Black is much underdeveloped, allowing white to take the initiative with 1.. c5 2. dxc5 dxc5 3. Qd6.

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From my experience, the ...c5 move should be analyzed case-by-case. The purpose and soundness of the move depends on the pawn structure, piece development and tactical opportunities around the board.

Thanks, I see now. This helps.

@ pfren

OK thank, I'm prepping for my match tomorrow.  I'll have a look at this again. For the second diagram I already prepped ...Nd7 first instead of ...c5.

Thanks, I see now. This helps.