20 MAG Bar Buy?

1. In very strong bull trend as shown in this slide, why is there no buy when the bears first touch the MA? Also - first close beneath, and first MAG Bar

I told you before, because the trend had already finished, at least momentarily, after the climax. 

1-2. BO above bull channel in the form of a microchannel and after more than 20 bars, so possible climax and you have to expect two legs s¡deways to down.

3. H1 but bear bar and after 3 bars without a bull bar, so sellers above for the second leg sideways to down coming from the climax.

4. Second leg down, so H2 3, and MAG bar setup but only a doji SB after strong bear bar, so better to wait for second entry or bull BO.

5. Instead, 5 reversed at EMA and broke out below, then the MKT plunged.

2. Starting at the left, where would you enter and exit

I can't see the bars to the left but once the channel begun and until the climax, any bull bar closing on the high was a buy, if you could bear the risk.

3. H1 but bear bar and after 3 bars without a bull bar, so sellers above for the second leg sideways to down coming from the climax.

What is your definition of H1?

 H1 (high 1)= the first time a bar has a higher high within pullback leg#1 of a bull trend.

The Bear bar (#3) has not made a higher high, however the next bar has, which should be an H1.

What is your definition of H1?

Not mine, Al's... but there is a confusion between the books and the course.

H1 (high 1)= the first time a bar has a higher high within pullback leg#1 of a bull trend.

The bar that goes above is actually the H1 entry bar (EB), but the signal (SB) bar is the previous one, the one I marked. At the beginnings, Al used to mark a setup either with the SB or the EB, nowadays he strictly uses the SB which, by the way, it is more convenient because it is the the bar that makes you place the trade (most of the time, there are bars that can be both EB and SB).

If not clear, watch again the related videos on bars and bar counting (#8 and #9) or, alternatively, go to the encyclopedia related section.

The slide below comes from the encyclopedia sampler in which all the setups are marked using the SB: 

Does more legs= higher probability? Example> H2 is higher probability than H1

And how do you know H2 or H3 will happen? What if you wait, but it's only an H1 and you miss the opportunity

It depends on the strength of the move. If it is very strong, a H1 can work but if not better to wait for a H2. Yet, if this second leg down is somewhat strong then you can wait to enter when a H3 forms.

Posted by: @w

And how do you know H2 or H3 will happen? What if you wait, but it's only an H1 and you miss the opportunity

Of course a trader will miss some high profit trades. You will eventually take the trades you judge as having better trading equations, in accordance with your experience, your risk appetite, your personality, and per the videos on Traders Equation.

Also bar counting-related opportunities consider context, as Ludopuig and the Bar Counting videos mention.

Hopefully this doesn't add a world of confusion, but its worth noting that the second two H2 buys on this chart below are also 'second entry buys'. This is the second buy where the entry price is higher than the first entry (or lower in the bear case) and the stop-losses for the first entry haven't been hit.

These are VERY good setups, even better than an H2 buy, because bulls are encouraged because they've got a second chance to enter therefore a new set of bulls jump-in and join the first lot who still are holding their long, and from the bear perspective the second entry buy represents failure of their short setup so bears will give up above the second signal bar.

I will almost always take second entries in a with-trend context and they're usually good for at least a scalp in a counter-trend context. 

They're covered in Video 08D - Second Signals and also in a bit more detail in Trading Price Action Trends Chapter 10 - Second Entires.

The slide below comes from the encyclopedia sampler in which all the setups are marked using the SB: 

Isn't the second H1 entry a second entry?

A H1 is the first attempt to resume after a pullback (indicated by bar going above pullback bar or inside bar), H2 is the second occurrence.   But H1/H2 could be bad entries/signal bar so you might not consider it an entry.  So "second entry" or "second signal" is the second occurrence of a reasonable entry, with the rules I indicated above.

Hope I've not confused things, but in a nutshell as well as being normal H2's, the last two of the examples in @ludopuig's chart were also excellent second entry buys aka second signals - these very very good entries.