Help me to understand the joke,please)

It is a joke about the stork which is a large white bird that brings babies home. This is what parents say in some English speaking countries to their young children when they ask where babies come from.

In the story the stork doesn't make mistakes and brings the right baby to the right house, but here he made a mistake. I think it could mean that the baby wasn't supposed to live, or could mean that he was adopted or maybe another meaning.

so in short, the joke is about the stork making a mistake and the baby who knew the right address :D

That is absolutely right @Patrick, and as @Aliph pointed out well, MSA material is either scarce, poorly written, or too local and in some cases all of that together. MSA doesn't have a formal body or institution that regulates it or collects it in a corpus say like Cambridge or Oxford do with English. Therefore, any advice or statements about 'proper' sources of learning Arabic (particularly from native Arabs) should be taken with a pinch of salt.

Many experts and linguists specializing in Semitic languages tell us that contrary to what most Arabic would have you believe, the Quran is not a good example for learning either MSA or Classical Arabic. It contains many words from proto-Arabic and other Semitic languages as Aramaic (Syriac), Hebrew and Farsi. Some of the Prophet's contemporaries seemed to have problems with understanding it as the it was different from the Arabic used in pre-islamic Arabia such as the Arabic used by AlMutanabi and other famous poets of his era (which the linguists tell us is a much better reference for Classical Arabic).

The lack of consensus over the meaning/interpretation of many verses of the Quran between the orthodox Sunni Quranic scholars, Shia/other scholars and secular linguists (many of them in the Islamic Golden age) has led to a lot of controversy which continues to this day.

Even the 'Arabic' words in the Quran, many are either too archaic or already obsolete that we have stopped using them completely or use them with a different meaning or connotation. The same goes for Classical Arabic of the poets which modern lay Arabs (myself) included don't understand without extensive and often word by word annotation as in school or college literature classes.

All in all, I would advice any learner of Arabic to focus on fluency rather than accuracy when learning MSA considering the unreliable sources, and learn to be tolerant of the ambiguity and simply accept the messiness of the situation!

@Aliph I think the reason Arabs 'seem' to be learning English and other European languages with ease rather than the other way around, is related to exposure (internet, academia, entertainment) and European Languages being taught as second languages in Arab countries (almost all study English, some French too).

You'd be amazed at how foreign English or other Romance languages feel to Arab speakers (excluding some loanwords, words in common), but since they are forced to learn them (school/work), they don't pay much attention to these differences in syntax or phonetics. This is in contrast to Europeans or other learners who are interested in Arabic out of passion, curiosity or any other personal reason.