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Always cook over low heat so it doesn't come to a boil.
Literal
Without-fail low-heat [with-で] not-boiling [purpose-ように] cook [imperative-こと].
A recipe instruction. 必ず ('without fail / always') emphasizes the non-negotiable nature. 弱火で ('on low heat') uses instrumental で. 煮立たないように ('so that it doesn't boil') is the negative purpose construction ~ないように. The sentence ends with 煮ること — the nominalized imperative, where こと at the end of a plain-form verb functions as a written command ('you are to do X'). This is a hallmark of instructional writing: recipes, manuals, rules.