Reactions of the carbonyl group begins with the formation of a bond between the carbonyl carbon and an attacking nucleophile.The nucleophile provides the electrons to form the new bond and the π bond of the carbonyl group is broken as it "gets out of the way." The electrons are moved from this π bond onto what was the carbonyl oxygen. A strong nucleophile can attack directly, without help from an acid catalyst. For a weak nucleophile, an acid catalyst is needed so that the carbonyl carbon is prepared to share a pair of electrons as a new covalent bond.

It make the carbonyl carbon more electrophilic by protonation of oxygen of carbonyl group

Reactions of the carbonyl group begins with the formation of a bond between the carbonyl carbon and an attacking nucleophile.The nucleophile provides the electrons to form the new bond and the π bond of the carbonyl group is broken as it "gets out of the way." The electrons are moved from this π bond onto what was the carbonyl oxygen. A strong nucleophile can attack directly, without help from an acid catalyst. For a weak nucleophile, an acid catalyst is needed so that the carbonyl carbon is prepared to share a pair of electrons as a new covalent bond.

I am agree with yogesh.

Lewis acid are generally used ,these will not affect the efficiency of nucleophile .it only activate electrophilc character of c=o bond.

Any reaction on carbonyl carbon is done either by increasing the electrophilicity of that carbon by having the oxygen protonated or by increasing the nucleophilicity of the incoming nucleophile. The problem in using the base is that it may react with aldehyde to give various side products (Benzoin, aldol condensations etc.) or the base itself may act as nucleophile. Hence acid is used as a catalyst in these cases. Though there is a possibility of reducing the nucleophilicity of amine, it won't have much effect since it is used as catalyst. Instead a lewis acid can be used.

Check the mechanism. You need to activate the carbonyl towards the attack of the amine; then you protonate (for example) the carbonyl.

For both basic or acid mechanisms occur via two effects, H, Na or K---O=C activation as well as ineraction of the acidic Na or K ions or proton with the highly donor lone pair present in recently formed imine. Hence by the two approaches you get the needed Schiff base or simpler imine.

Nevertheless in any step avoid water sources by the use of Dean-Stark trap and solvent reflux (physical displacement of water) or via chemical desiccators.

By the way you know ,condensation reaction under drying condition can be done without any acid or base to preparing schiff base ,but need more time to ended .

Ya i know dry condensation reaction for preparation of azomethines but at the same time answer to your question if acid is used mostly schiff bases are susceptible to hydrolysis and are not very stable in acid conditions

If product is obtained in crystals form with delicate tiny tinder needle like shape then what are the possible ways to collect them without product loss?when amount is too small say 0,032g

look Aisha : you can collect your product by Freeze drier apparatus ( vacuum with cooling ).

Schiff bases are synthesized either in the presence of acid because the nucleophilic attack at carbonyl group needs polarisation of aldehydic carbonyl group so that amino group will attack carbonyl carbon.