Hi. I want to ask when is it required to use 'la' before manzana? Is 'el caballo come manzana' a wrong sentence?
Yes, the word ‘manzana’ is a [feminine] count noun, not a mass noun, so *‘El caballo come manzana.’ = *“The horse is eating apple.” is an ungrammatical sentence. If it's clear which apple the horse is eating, you use the [feminine singular] definite article ‘la’: ‘El caballo come la manzana.’ = “The horse is eating the apple.”; otherwise, you use the [feminine singular] indefinite article ‘una’: ‘El caballo come una manzana.’ = “The horse is eating an apple.”. And if the horse is eating more than one apple, you use the [feminine] plural definite article ‘las’ for specific apples: ‘El caballo come las manzanas.’ = “The horse is eating the apples.”; and optionally use the [feminine] plural indefinite article ‘unas’ for unspecified apples: ‘El caballo come (unas) manzanas.’ = “The horse is eating (some) apples.”.
In contrast, with a mass noun like ‘paja’=“hay”, the definite article is optional, the indefinite article is normally never used, and the plural noun is normally never used. So if it's clear which hay the horse is eating, you use the [feminine] definite article ‘la’: ‘El caballo come la paja.’ = “The horse is eating the hay.”; otherwise you don't use any article: ‘El caballo come paja.’ = “The horse is eating hay.”
In English, you wouldn't say, " Horse eats apple". La/El = The", "The horse eats the apple." Is the best translation. :)
Btw if you are having trouble transcribing do this: put the sound to phones speaker, crank up the volume a bit and turn on voice recognition. Press play to hear the sentenve aloud - the text will appear!