If you're trying to measure a degree of comparison, then use "as": He is trying as hard as his sister to make friends. If you're trying to simply make a comparison, use "like" as a preposition: Like his sister, he is trying to make friends.
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If you're trying to simply make a comparison, use "like" as a preposition:He is trying as hard as his sister to make friends.
Like his sister, he is trying to make friends.