Some students understand this idea intuitively. For those who don't, I have a crucial message: Don't fight the language! It took centuries for the language to develop and it's not going to change just because you feel more comfortable expressing something in a way that is more similar to your native tongue.
So if you hear a native speaker say something a certain way, assume that that is the only way to express it until you hear otherwise from another native speaker. For instance, if a native speaker says she's sitting and in your language it's different, e.g. she's seated (está sentada), assume that in all cases in English we would express the idea using the present participle sitting of the verb to sit and not the past participle seated of the verb to seat. But if you board a plane and the flight attendant says all passengers must be seated, then you'll know that it is possible to use the past participle of seat in English and that it really just depends on the context - in this case a more formal one.
I always tell students that language is not like mathematics. Language is rarely 100% logical. It's often messy; sometimes flexible, sometimes not. At the end of the day, it is what it is. And that goes not only for semantics, but also for syntax, grammar and phonology. The goal should be to accept it readily and repeat it. Use newly acquired expressions frequently at first, just as a parrot would repeat phrases it learns, and then remain open and ready for the next expression to come your way.
If a native speaker says he's 64, that means he's 64 years old. Note that the verb to be is used to express someone's age. Your language may use another verb, such as have. Don't force your language onto English. If you say he has 64 years, a literal translation from many Latin-based languages, you're going to sound very odd and may not be understood. By the way, did you catch the other detail? In English, it is not necessary to say years to express someone's age. But if you do, you need to say years old, not simply years. There's a prime example of staying open and attentive to every detail.
While it's important to pay attention to the details, there simply isn't enough time to question every little detail. When students get hung up on the way the foreign language expresses something, they miss the next opportunity to learn more of the language. So, stay open, listen carefully and be a parrot! That's how successful language learners do it.
It helps to think of language as short segments or phrases, not so much individual words. Treat these phrases like gold nuggets that you mine from your foreign-language conversational partner. Each is precious and should be stowed away to be used later; hopefully soon afterward, so that you can solidify it in your memory.
When learning English, in particular, watch for how the verbs are used with particles. These particles look like prepositions, but they're not. They are there to transform the meaning of the main verb. We call these phrasal verbs, or two-part or three-part verbs. So, for example, while the verb work may mean trabalhar or funcionar in Portuguese, the verb work out would mean malhar or resolver, depending on the context. And, speaking of context, always learn a new word or phrase noting the context in which it was used.
The meaning of a word in your native language often does not have an exact equivalent in the foreign language. Sound in English could mean som in Portuguese, but it could also mean sólido, seguro or íntegro, as in this building in sound. While welcome means bem-vindo, the expression you're welcome would mean de nada. If you analyze the word welcome, you see that it is a combination of well and come, very similar to bem-vindo. When we say you're welcome, what we are really saying is something like fica à vontade, which is what we use to say de nada, por nada or não há de quê. To take all this in, your mind needs to remain flexible.
If thinking in the above way helps you to remember an expression, great. Use it. But often times, there is no way to make any sort of connection in meaning. When a salesperson in a store in Brazil greets me with pois não?, I know he/she means may I help you, but there's no way to make sense of the expression pois não?, so I just remember it as is, without questioning it any further. Then, most importantly, I use it in the same way and context that I heard it.
Watch out for false cognates - words derived from the same root but whose meanings have diverged. Take Portuguese and Spanish, for example - two languages that are quite similar from an etymological standpoint. The word propina exists in both languages, but it is now a false cognate. Many centuries ago, it likely had the same meaning when it was introduced by the Romans or Greeks (the exact origin doesn't matter). But over time, its usage changed to the point where now una propina in Spanish means uma gorjeta in Portuguese and uma propina in Portuguese means una coima in Spanish. Try explaining that to the police in a sticky situation!
The creation of new vocabulary, and the change in meaning of vocabulary and phrases between languages that historically have a common root, happens in part as a result of the cultural needs of the population that uses the language. Eskimos, for instance, have numerous ways to express different types of ice. They need to be able to express ice in its various forms in order to survive in that climate. But it goes beyond mere survival. When you see something every day, you begin to notice the intricacies of it and that leads to the desire to more accurately define and express it. For most of us who live in warmer climes, a handful of words to describe ice suffices. But for the Inuit of Canada, who appreciate ice to a much larger degree than we do, discerning the many differences in the quality of ice has led to the creation of more than 53 words for it. And in the northern parts of Scandinavia and Russia, the Sami people use around 180 words related to ice and snow!
You can think of the meaning of words as overlapping circles. In the image that follows, each circle represents the meaning of one word. The space in between the circles represents an aspect of reality that has not been defined by the language. Interestingly enough, when a language doesn't define something, the undefined reality often goes unperceived by the speaker of that language.