In your mind there is a box labeled "English". If you speak another language too, then there is another box labeled "Spanish", or whatever other language you speak.
You carry that box around with you, and it's easy to carry everything that's inside of it, because it's all packed into that box.
When you speak that language, you open the box, and pull out the words and phrases you need to say what you want to say.
But those words and phrases are not randomly strewn about like a junk box, but rather they are organized in smaller boxes.
Within the main box, the next set of smaller boxes is labeled by topic, or situation. For example, there is a box labeled "greeting", and one for "introductions", and one for "weather", and so on and so forth.
After opening the main box, you open a smaller box for the situation you are in, or the topic you're speaking about.
Within each "Topic" box, there is a set of boxes that are each labeled with a verb. For example, in the "introductions" box there is the verb "I am", and in the "weather" box there is the verb "it's".
And inside each "Verb" box, there is a set of boxes labeled with the vocabulary that completes the sentence that the verb box begins. Such as, "Tony", and "from Chicago", and "a teacher" -- which are inside the "I am" verb box -- and "warm", "sunny", and "nice out" -- which are inside the "it's" verb box.
You've been organizing your "Language" box since the day you began learning that language (through listening and speaking, and later reading and writing).
The process of learning a new language is organizing a new box. You put in boxes for topic/situation, and in each of those boxes you put in verbs, and in each of those boxes you put in vocabulary.
And then you can carry that language around, and you'll always know where to find stuff.