Spins spin coupling on proton NMR for an aldehyde?
I cannot decide whether the H in the aldehyde group would be split into a doublet or remain as a singlet [&hellip;]


What would be the argument against $^3J$ coupling between these protons? The aldehyde proton will appear as a doublet. 

Don't forget that coupling works both ways! $\ce{(CH3)2C\color\red{H}-}$ will (at least) split to a doublet too. Coupling with this proton will also  split the signal for the methyl protons to a doublet.

What does this mean for the proton at C-2? 

Formally, a doublet (from coupling with the aldehyde proton) of a septet (from coupling with six magnetically equivalent methyl protons) is to be expected. 

In really, you will observe a smaller number of lines! I suggest to try this with a pen on squared paper to figure out what happens when the two coupling constants are identical or one is twice as big as the other.