"She insisted on eating pasta."
Translation:Ella insistió en comer pasta.
en comer = on eating. de comer = eating. Why ''en comer'' and not ''de comer''? I think it has something to do with the difference between how the present participle is used in English (-ing verb form) after prepositions whereas in Spanish the present participle is not permitted after a preposition. Comer then translates as just ''eating'' not ''on eating'' because you have to use a preposition before the Spanish infinitive. The Spanish preposition that translates to ''on'' is ''en''. ''De'' = from, of, by, to depending upon the context. Sorry, I probably confused more!
why not 'la pasta' as in "ella insistió en comer la pasta" ? she insisted on eating THE pasta. IMHO would seem to fit the preterite better as it pinpoints a moment in time when the pasta was available... ella insistió en comer pasta'? she insisted on eating pasta does not appear to contain any explicit reference to an occasion in the past and could easily be construed as at any time in the past up until the present. (imperfect). Note: the present = the period of time now occurring, now, today, the present time, the here and now, this day and age, the present moment, (for) the time being...she insists on eating...
I think one of the points being made in this set of exercises is when it's ok in Spanish to drop the definite article. It's a painful process trying to work out the rules just from DL. Better to read up on it. Here's one source: https://www.thoughtco.com/how-to-use-definite-articles-3079100
Also the questions about why it's "en" not "de" [comer]. That's because "insistir" takes "en" + verb infinitive - 'comer' in this case - to mean 'insist on [doing something]'.
Other verbs take "de" e.g. "dejar de" + verb infinitive - 'stop [doing something]
Other verbs take other prepositions or none at all. And some take the gerund (-ndo form) without a preposition. Yes, it's a nightmare! But here's a great link someone posted in another thread which sets it all out: http://users.ipfw.edu/jehle/courses/VRBSPREP.HTM