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I started using a dictionary again when I used in my shcool days.
When I used to refer to it, drew a line around the word.
The first time I looked up a new word in the dictionary, I drew a red line under it. The next time, on the left side of it, the third time, upper, finally the word was framed completely.
Now when I study English, I refer to the dictionary all the time, and I notice the words are almost always with lines.