Let's learn to speak Spanish: What do you mean . . 'you couldn't do it'?

A simple enough phrase, 'You couldn't do it', but there are at least 12 different ways to say it in Spanish (16 if you're in Spain, not Latin America)

Let's start by looking at what the phrase actually means.
A common complaint, when people begin to speak Spanish is that the same word or phrase can mean different things, depending on the context.
Well, it's the same in English.

We'll start with the simple part, the word 'You'
There are four options here:
'Usted' - polite form of 'You' singular
'Tu' - familiar form of 'You' singular
'Ustedes' - plural of 'You'
'Vosotros' - plural of 'You', familiar form. Not used in Latin America

So, having determined who we are speaking to, now let's figure out what we're actually trying to say.
Is it 'You were not able to do it'
or 'You would not be able to do it' ?

Let's go for 'You were not able to do it'. Simple. Past tense.
Ah, but which one?
Is it Preterite, as in 'You were not able to catch the bus at 6 o'clock last night', or Imperfect, as in 'We played chess every night for a year, but you were not able to beat me'

OK, let's try the other option 'You would not be able to do it'
Is it 'You would not be able to do it, without my help': Conditional?
or 'I built a wall, in order that you would not be able to get in' : Past Subjunctive? ( following the construction 'para que')

Well, that gives us four (or three) different Persons and four different Tenses or Moods (Subjunctive isn't really a Tense)
Multiply them together, and you have 16 (or 12) different translations of an apparently simple English phrase.

Without wanting to complicate things, I'd best mention that there is another Past Subjunctive conjugation that you might see written but is rarely spoken, see the conjugation chart at the bottom of the page, which would actually bring our total up to 20!

How about some examples?

'Usted no pudo hacer un pastel anoche' - You (formal) couldn't make a cake last night (Preterito. Single action in the past)

'Tu no podías hacer un pastel jamás' - You never could make a cake (Imperfecto. Action repeated in the past)

'Ustedes no podrían hacer un pastel sin mi ayuda' - You (plural) couldn't make a cake without my help (Condicional: Would not be able)

'Escondí el azúcar para que no pudierais hacer un pastel' - I hid the sugar so that You (familiar) couldn't make a cake (Pasado de Subjuntivo: following 'para que' : 'so that you would not be able')

So, the next time you're struggling to figure out exactly how to translate a phrase, in English, to Spanish, stop for a moment, and figure out exactly what it is you want to say in English, and it might help you get to the correct construction in Spanish.

See?
I knew you COULD do it, really.
¡Hasta pronto!

P.S. some conjugations for you.