I was wondering if there was an explanation for why Data from ST:TNG couldn't use contractions?
Prior to the 12th episode of the series, Datalore, Data did use contractions. In Datalore, his tendency not to use them was pointed out by Lore:
However, at the end of the episode, Picard asks:
And Data replies:
After this (fairly early) episode, Data rarely used contractions again (so it was somewhat of a retcon). In the cases where he does, it's generally because he's playing another character (e.g. on the holodeck), or to demonstrate an enhancement to his original programming. For example, in Insurrection, after he has received his emotion chip, he says to a child:
So Data could, in fact, use contractions, but generally did not. The suggestion is that his original programming did not provide enough information to use them spontaneously in everyday speech. Perhaps also, given that Lore had mastered contractions, this was a deliberate limitation in Data by Dr. Soong, like excluding emotions, to make the android more acceptable to others. (And one that, like emotions, was intended to be overcome once Data was older).
Lal, the android Data created in The Offspring, did master contractions:
More information, and in particular more examples of when he does use contractions, on Memory Alpha's page on Data.
I'd like to point out a possible alternative explanation, although I do not believe it to be official canon: English is hard.
Data had to learn how to do almost everything. While he almost certainly had basic English programmed into him, I find it more likely that contractions weren't necessarily programmed in, beyond (possible) the most common ones. The rest, he's had to actually learn, and is having difficulty with.
One reallife example is human children and English conjugations (This is an exaggerated/paraphrased example; I couldn't find the article I read this in):
The "forgetting" is because instead of just repeating the phrase by rote, they're trying to figure out why it's "want" instead of "wants". It's a step towards ingraining the rules of grammar so that it's automatic.
I believe that throughout TNG, Data is somewhere between points 1 and 2. Soong had programmed in several contractions for Data to make use of, automatically replacing words with their contractions if he was going to say them aloud. Data had, by the time of TNG, been unconsciously replacing some of those protocols with the more formal form, in an attempt to learn the patterns involved so that they could be applied more generally.
Point 1 is supported in that Data's contractions are often either some common phrase ("I'm fine."), repeating something someone else just said (where he doesn't have to figure out anything), or manually programmed into himself ("Honey, I'm home!").
Point 2 is supported by Data's use of the formal form becoming more common as the series progressed. After Lal pointed it out to him, it could even have become more ingrained, something he was doing unconsciously to be more unique. (Part of what drove Data was to become more human, and one of the key qualities about humans is that we're all different from each other, while machines are not)
Point 3 is supported in the series finale of TNG, in the timeline where Picard is in the future - Data can fully use contractions in everyday speech.
There's an instance I haven't seen on ANY comprehensive list of Data's "contraction slip-ups", in season 6 ep 21 "Frame of Mind" around 27 seconds into the episode where Data says, "You mean you don't think you should stand trial..."
This contraction really got me thinking, since it's not a sentence Data's coming up with on his own. It's a line in a script he's reading. Any thoughts on how his brain would work? Obviously he can memorize a string of sounds and repeat it, even if those sounds are contractions. But if he can master THAT, why can't he use contractions as parts of common whole phrases?