We know that Grignard reagent is strongly basic. It is stronger than NaOH.  

We also know that sodium hydroxide is just enough to abstract a proton from active methyl group of acetaldehyde to carry out aldol reaction.

Now my point is, if we use Grignard reagent, acetaldehyde should undergo aldol reaction instead of Grignard reaction.

Any comments?

18 hours ago, adianadiadi said:

Now my point is, if we use Grignard reagent, acetaldehyde should undergo aldol reaction instead of Grignard reaction.

Clearly, no it "shouldn't", or it would.

It seems that the aldol reaction is faster.

Or maybe the proton transfer is fast, but effectively reversible.

NaH is a non-nucleophic base, so it won't attack the carbonyl carbon. Grignard reagents are, so they do.