user8737: i'm wondering why black doesn't play c4 and b4 after b5 and c6 in this game. Wouldn't that be a tempo, i.e. the bishop on d3 had to move? Additionally it would gain black central and on the queens site room advantage, right? So why doesn't black play that?

user7500: If white plays e4 with out black having played ...c4, then black can trade the central pawns with cxd4 to reduce white's central control.
[StartFlipped "0"]
[fen "r1bq1rk1/p2nbppp/1p2pn2/2pp4/3P1B2/2PBPN1P/PP1N1PP1/R2Q1RK1 b - - 0 1"]

1... Bb7 2. Re1 Rc8 3. e4 cxd4 4.cxd4 dxe4 5.Nxe4 Nxe4 6.Bxe4 Bxe4 7.Rxe4 Nf6 

(Black can play 2...Ne4, which is stronger, but that's a different story.)
However, when black has played ...c4, then e4 is especially strong because when black takes with dxe4, white's increased control of the center is difficult to challenge, as black lost the option of cxd4. 
[StartFlipped "0"]
[fen "r1bq1rk1/p2nbppp/1p2pn2/2pp4/3P1B2/2PBPN1P/PP1N1PP1/R2Q1RK1 b - - 1 1"]

1... c4 2. Bc2 b5 3. e4 dxe4 4. Nxe4

If black does not trade with ...dxe4 then white will play e5 and gain an easy attack on the black king.
[StartFlipped "0"]
[fen "r1bq1rk1/p2nbppp/1p2pn2/2pp4/3P1B2/2PBPN1P/PP1N1PP1/R2Q1RK1 b - - 1 1"]

1... c4 2. Bc2 b5 3. e4 Bb7 4. e5

Black can try to use his queenside space advantage and play for b5-b4xc3 to target c3, but this plan is slow and black is likely to get mated if he tries this.

8737: <p>i'm wondering why black doesn't play c4 and b4 after b5 and c6 in <a href="https://youtu.be/8q28BudG2eU?t=117" rel="nofollow">this</a> game. Wouldn't that be a tempo, i.e. the bishop on d3 had to move? Additionally it would gain black central and on the queens site room advantage, right? So why doesn't black play that?</p>
 7500: <p>If white plays <code>e4</code> with out black having played <code>...c4</code>, then black can trade the central pawns with <code>cxd4</code> to reduce white's central control.</p>

<pre><code>[StartFlipped "0"]
[fen "r1bq1rk1/p2nbppp/1p2pn2/2pp4/3P1B2/2PBPN1P/PP1N1PP1/R2Q1RK1 b - - 0 1"]

1... Bb7 2. Re1 Rc8 3. e4 cxd4 4.cxd4 dxe4 5.Nxe4 Nxe4 6.Bxe4 Bxe4 7.Rxe4 Nf6 
</code></pre>

<p>(Black can play 2...Ne4, which is stronger, but that's a different story.)</p>

<p>However, when black has played <code>...c4</code>, then <code>e4</code> is especially strong because when black takes with <code>dxe4</code>, white's increased control of the center is difficult to challenge, as black lost the option of <code>cxd4</code>. </p>

<pre><code>[StartFlipped "0"]
[fen "r1bq1rk1/p2nbppp/1p2pn2/2pp4/3P1B2/2PBPN1P/PP1N1PP1/R2Q1RK1 b - - 1 1"]

1... c4 2. Bc2 b5 3. e4 dxe4 4. Nxe4
</code></pre>

<p>If black does not trade with <code>...dxe4</code> then white will play <code>e5</code> and gain an easy attack on the black king.</p>

<pre><code>[StartFlipped "0"]
[fen "r1bq1rk1/p2nbppp/1p2pn2/2pp4/3P1B2/2PBPN1P/PP1N1PP1/R2Q1RK1 b - - 1 1"]

1... c4 2. Bc2 b5 3. e4 Bb7 4. e5
</code></pre>

<p>Black can try to use his queenside space advantage and play for <code>b5-b4xc3</code> to target <code>c3</code>, but this plan is slow and black is likely to get mated if he tries this.</p>
