One of the most confusing things in new language acquisition is learning colloquial usages, and one of the hardest things about colloquial expressions is the use of prepositions. Yeah, I know ya'll know what I'm talking about.
Let me break it down: some English examples could be, "You are from Boston," "Come to my house," and "I live in Eritrea." Once upon a time, your average ten-year-old could parse these sentences and even identify the cases of the nouns, but, dear 21st-century readers, if you're not tracking, the prepositions here are from, to, in.
If you want to translate these sentences literally, you are assuming that your target language expresses these ideas using direct equivalents of from, to, in, respectively.
Except, they often don't.
You'll find translations in language dictionaries telling you, as closely as possible, what prepositions correspond to our English ones. The problem is, we associate a whole array of meanings with each of our prepositions, and the foreign counterpart may contain an overlapping but different assortment of usages.
Think of it like this: Sally and Jane each have a kit of art implements. Sally's contains crayons, markers, and paints, while Jane's has crayons, colored pencils, and pastels. In Sally's world, "implement" has three distinct meanings that only overlap with Jane's meanings 33% of the time.
You can think of all prepositions in all languages as little Venn diagrams: circles of content that overlap, each with several of another language's prepositions. There's just no simple translating mechanism for that.
One solution is to painstakingly memorize all possible meanings of da or en. Or, you could learn many expressions over time and eventually sort of absorb the various uses of the prepositions therein. My solution speeds up this second approach.
I've found that the easiest way to learn prepositions is to mentally cache them ALL and avoid ever really translating them into English at all. See, all prepositions are just descriptions of relationships. If I am on a boulder, I could misspeak and say that I am under, in or between the boulder, and based on context or visual clues or logic you could mentally correct me. Each of those words tells of some relationship, and without knowing any of those words you could still see or deduce the idea of on.
So when I read a foreign phrase, I translate it like this: "John was [preposition] the house," or "I can see [preposition] the window." I translate everything BUT the preposition, and leave it in its original form. This way I'm just seeing the context, not translating.
My [lay] reasoning is, an English translation brings with it a whole set of assumptions about which usages should be grouped together. If I translate each usage, I have to mentally knock down and rearrange the boundaries of those categories as I go. By NOT translating the prepositions, I can use context to slowly, organically create a NEW mental category for grouped relationships.
Easy peasy, lemon-squeezy.