Words are the fuel of language. The number of words you are familiar with determines your language abilities. The more words you know, the better you are. Put in numbers, this statement reads as follows:
15,000 > 10,000 > 5,000 > 2,000 > 1,000 > 500
Between 2 and 18 years, you learned 10 new words every day. Later, at work or at university, you enriched your brain vocabulary with thousands of technical words. Now, after decades, you know more than 50,000 words of your native language. Words are the hard stuff of language; in comparison, learning grammar is a finger exercise for pre-school children.
To be comfortable in another language you need roughly half of the words you possess in your native language – 25,000. As about 40 percent are variants of other words and can be easily inferred, a good estimate of truly unique words you need to start with is 15,000 words. This is a huge number and double what you are expected to learn in 8 years at school. Fortunately, you do not always have to learn them all. Take the word evolution. In Spanish, Italian, and French, the word translates into evolución, evoluzione and évolution. As you can see, many words are almost identical between some languages and come with just slight differences in packaging. Once you understand the rules that govern these differences, you have immediate access to thousands of words.