In many TV Shows and movies I've watched, person A is saying something in language X, and person B understands it, but cannot speak language X. I looked this up on the Internet and found an article about this.
Is it really possible that one can understand a language, but can't speak it?
Yes, it is possible to understand a language without speaking it. This often happens in when children are brought up bilingually. If children are exposed to two languages, but one is dominant - that is, they hear it a lot more than the other(s), they will tend to use this language. If everyone can understand them when they use the dominant language, they will not see a need to learn to use the non-dominant language. It is only with a lot of experience using the language that you can actually use it.
For example, if a child grows up in English-dominant North America, but they have a grandmother that uses a heritage language, the child may never learn to speak that heritage language. Nevertheless, they will understand it. I've known some Korean-Americans who can understand Korean quite well, but when they come back to Korea, they are (mostly) unable to speak the language.
Yes, it is possible, at least partially. This is called Mutual intelligibility.
You can find a list of mutually intelligible languages.
As for the only known example to me, Spanish - Italian, it's also a question of the used vocabulary in order to maximize the similarities. The usual term of one language can be the unusual/formal version in the other.
Yes, it is perfectly possible. There are huge differences in active and passive language performance. Language teaching nowadays focusses on active performance as well as passive performance, but sometimes (e.g., in the case of a dead language like Latin) the passive performance is all what counts.
A lot that is conveyed in communication happens through what is called paralinguistic features. One paralinguistic feature is body language which can further be divided into facial expressions, gestures, posture, and proximity.
I think most of us have experience knowing that someone was telling us to stand up or sit down based on body language alone. Body language is especially useful in conveying simple commands and responses to questions. For abstract conversation, body language is still useful but not as helpful in understanding what is being said.
I know that anecdotical evindence is no scientific evidence, but whatever …
It is possible, and it is even possible to narrow down the language that is spoken that the person does not speak to a single one without any additional information. The anecdote is as follows:
My then girlfriend and I were on a train and heard (but due to the layout of the seats did not see) a set of two or three ladies speaking in a language unknown to us. I didn’t recognise words, only went by sounds and placed it in Eastern Europe. My then girlfriend noticed that it sounded Spanish-ish but wasn’t Spanish and with time passing recognised (and understood!) more and more sentence fragments despite having learnt Spanish for only one or two years (and having no knowledge of Italian or French, only of Latin and English at the time). Thus, she narrowed the language down to Romanian, got up, asked the ladies what they were speaking and it was indeed Romanian.
