[K.z_z]

Reaction of HCN with an aldehyde has higher yield than HCN with a ketone due to steric strain around the ketone. Then how about reaction yield of HCN with an aromatically conjugated aldehyde (like Ph-CHO) comparing with the previous two reactions?
What I think:
The yield be lower than just with regular aldehyde and higher than with ketone. Since the phenyl group is Electron donating, thus making the carbonyl carbon less Electrophilic. Tell me if I am right or wrong, thank you.

[Babcock_Hall]

What makes you think that a phenyl group is electron-donating?

[K.z_z]

When we learn addition to a benzene, the professor listed phenyl group as a weak activator which means it is electron donating. I am not 100% sure if that also applies here?

[Babcock_Hall]

This is not my strongest area of chemistry, and most of my books are not presently available to me.  However, my understanding is that the phenyl group is slightly electron withdrawing in terms of the inductive effect, which is why benzoic acid is a stronger acid than acetic acid.  Directly attached groups can undergo resonance delocalization with phenyl rings.  I am not sure how all of this will play out with respect to your question.

[Dan]

When we learn addition to a benzene, the professor listed phenyl group as a weak activator which means it is electron donating. I am not 100% sure if that also applies here?

The Hammett constant for Ph σ_p = -0.01*

If you are going to use this value for comparison to an aliphatic aldehyde, you ought to compare it to an aliphatic group (an alkyl chain). How does it compare?

While the activating/deactivating properties of these substituents in electrophilic aromatic substitution may provide some information, I think that it would be limited to the comparison of different aromatic ketone (e.g. predicting reactivity differences between 4-methoxybenzaldehyde and 4-bromobenzaldehyde). I would not recommend using them to predict the reactivity aromatic vs aliphatic aldehydes. There are too many other factors at play (such as sterics, as you mentioned with reference to ketones, and conjugation effects).

One data source you ought to look into is the hydration equilibria of aldehydes and ketones.**

*Source here
**This paper may be useful

[orgopete]

Reaction of HCN with an aldehyde has higher yield than HCN with a ketone due to steric strain around the ketone. Then how about reaction yield of HCN with an aromatically conjugated aldehyde (like Ph-CHO) comparing with the previous two reactions?
What I think:
The yield be lower than just with regular aldehyde and higher than with ketone. Since the phenyl group is Electron donating, thus making the carbonyl carbon less Electrophilic. Tell me if I am right or wrong, thank you.

I agree that it can be difficult to ascribe a single property to some substituents. Aromatic aldehydes do react more slowly and benzoic acid is a stronger acid. The IR of benzaldehyde shifts to lower frequency. Most textbooks and your professor will argue this demonstrates electron donation by the benzene ring. Your suggestion is in line with this thinking and the answer I'd use on an exam.

Babcock and Dan may have some additional nuance to this question, but I am not familiar with it.

[Babcock_Hall]

Clearly cyanide ion can add to benzaldehyde, which is the benzoin condensation.  However, cyanide ion is the catalyst in this reaction, and cyanide is very probably the stronger nucleophile.