The issue is 'a' vs 'the' vs nothing:

    he went to the school
    he went to a school
    he went to school

'Chocolate' is another example of this, where the issue extends to the use of words like 'some', 'how many', and 'how much' (countable vs. uncountable quantities). Even if a person understands these concepts, it's still difficult to propagate it through a sentence so that everything type-checks (subject-verb agreement, etc), especially if you're coming from a language that doesn't have this kind of redundancy. It gets even harder with hypotheticals, where sometimes one might switch between both singular/plural and non-specific/specific mid sentence, all while English lacks gender-neutral singular third-person pronouns.

On top of this there are cases where the use of one over the other really just comes down to philosophy (whether native speakers of a language consider something distinct, concrete, or countable, or not). Similar issues come up with prepositions - which preposition goes with which verb differs even when describing the same concept in two very similar languages. (Best example I can think of at the moment is that in Japanese, people say "I bought it to her" instead of "I bought it for her"; I know English<->Spanish has similar issues, but my Spanish is rusty)

English is not my native language, but I don't think I have problems applying the work the. So I find that hard to imagine. However, my native languages are Dutch and German, which are probably the closest two languages to English in terms grammar. So, that could be a possible explanation.

I read that children learn to pronounce the th sound quite late. And this is generally a give-away for of non-native speakers. Could it be that you are confusing this with that ?

However, I was referring to the actual usage of the definite article, not its spelling. I think the problems here are of two different kinds:

Those who come from languages where there is no definite article (e.g. Chinese, Turkish) struggle to understand the correct usage. This is one of the most common glitches I hear from my Mandarin speaking colleagues (in addition to omitting plural forms).

Not all IE languages have articles, i.e. Hindi, Russian, Latin. Simplifying thing, one may say that English is a curious mixture (or bastard :-) of French and Anglo-Saxon, both of which has articles. It shed a lot of its grammatical rules over the centuries (e.g. noun declension, verb endings) but still carry vestiges, like the appendix, which is what confuses other IE speakers. These errors, tend to be the opposite of the Mandarin case, i.e. the over-regularization of the usage. My wife, who is near-native in French, does this frequently in her English usage.

This brings to mind a slightly different topic: The prediction of gender for (genderless) words adopted from English, i.e. if you are German and see the word iPad for the first time, which gender do you use? This study (http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.132...) tackled this problem for Scandinavian languages and found that it is quite difficult, they get ~70 accuracy.

The working class accent for where I'm from (Dublin, Ireland), prounounces "th" as "d", so "the" can be said "de".

A german friend advised his mother (who's is an english teacher (in germany)) to teach her students this, since it's easier for germans to pronounce!

Indicative that: "Which apple do you want?" "That apple." "This apple?" "No, that apple."

Causative that: "Why did you go to school?" "So that I could study Japanese."

Antecedent that, where that takes the place of some concept which has come before: "Did you study Japanese?" "That is a given."

"Never going to see it outside of literary constructions" that: "That which cannot be seen is more real than that which cannot be felt."

Adjectival-clause-introducing that: "We are looking for two students that are studying language pedagogy."

etc, etc

English grammar, fun stuff.

Fascinating stuff!

'Which' wouldn't be used here because 'are studying language pedagogy' is restricting/filtering 'students', and so it's essential to the meaning of the sentence. 'Which' is used for clauses that add non-essential information.

    My shirt that is red is upstairs.
    My shirt, which is red, is upstairs.