I would like to understand the using of the word focus. Do I need to use focus on or focus? Which one is correct?
The focus of this section is on introducing something.
The focus of this section is introducing something.
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In your case, the word focus is a noun. If you are not sure about the usage, always check dictionaries. Check the examples. Cambridge bolds the prepositions that are mandatory to use in almost all words' definitions.
Focus as in Cambridge reads:
- the main or central point of something, especially of attention or interest*
The example following the definite uses of or on depending on the contexts. In your case, it should be used with the preposition on.