"How"/ "when"/ "what"/ "where"/ is part of the sentence structure. We can't omit it. Not only did I learn how to use these tools, but I also acquired the ability to evaluate both their advantages and drawbacks.
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kindaichi000Why do I have to add "how? Is it a must? Is it gramatically wrong to omit that?I would not say that you have to use "how", but it emphasizes that you learned the (correct) way of using the tools, so it sounds better (to my ear) with "how".