In English, to split hairs means to argue about unimportant details, not to do something with too much care. Is this what you meant? I can't be sure. If you said "You are just splitting hairs" then you would be accusing someone of disagreeing with you about a very trivial detail, something that doesn't matter in the grand scheme of things. But to do something with too much care is not the same thing at all. Does the French phrase mean to do something with too much care? Or to quibble about petty distinctions? Or both?
I dunno...I think rebekasto has a point: it seems to me that making unimportant distinctions is not the same as doing something with an inordinate amount of care (even if they have overlap in some cases). But I think sloggo has a point too, since it could be that what is "hair-splitting" to one person is, to another person, of fundamental importance in not holding muddled views (differentiates experts (and geeks!) from those with casual interest). The point is that "splitting hairs" definitely means making unimportant distinctions as its primary meaning; the question is whether the French idiom has the same meaning as "splitting hairs."
This is your sentence:
-il y a une bombe là-bas
There would need a space after the sign [-] and you should capitalize "il" to "Il". And, maybe you might add a full stop at the end, then a blank line for clarity in reading.
What I have just done is "couper les cheveux en quatre": going into unnecessary details, since I have perfectly understood your sentence.
Imagine that you are at a community meeting. The topic before the town Safety Committee is that some residents want a traffic light at a busy intersection. Imagine that you are one of the concerned residents. You are explaining to the Safety Committee how the traffic light will be paid for without costing too much. One of your neighbors interrupts and expresses concern. He wants to call it a "stop and go light" and not a traffic light. You turn to him and say, "Let's not split hairs."
It's unimportant what it's called at the moment. THE GOAL IS TO GET THE LIGHT to control traffic and enable residents to cross the street safely.
At this point what the neighbor wants to call the light isn't as important as getting the light. It really isn't on topic either.
Once the residents have the light your neighbor can call it whatever he wants.
I just checked on that and the length of the hint is limited. I.e., there is not enough space to enter the whole idiom. Now that we have been exposed to it, perhaps we will remember the expression: Ne coupons pas les cheveux en quatre (or this alternative) "Ne cherchons pas la petite bête". http://www.wordreference.com/enfr/split%20hairs
It is an idiom because the sentence means something other than the literal meaning. It's not about hair(s). An idiom: a group of words whose meaning considered as a unit is different from the meanings of each word considered separately. The imperative form of the first-person plural means "let's" + verb. By adding the negative "ne...pas" around the imperative verb, it becomes "let's not" + verb. That part is direct...and needs to be. When adding the reference to cutting hair into quarters, it is no longer literal, but figurative, i.e., an idiom. English speakers don't say "cut hairs into quarters"; we say "split hairs" which is the comparable idiom in English.
`Be careful because the sound is wrong.
It sounds like "Ne coupcons pas les cheveux en quattre" but it should be more like this:
You could easily come across the situation in real life where there is an idiom or phrase which cannot be translated well directly, you'll need to think on your feet in such situations to understand the meaning and translate it well. So for some it is good practice for those situations.
I have no objection to learning idiomatic expressions. My complaint is with the methodology. Why force us to try and parse out colloquial meanings? Please, just give us the terms to memorize. The struggle I, and probably others, have with this instruction method is not necessary. It does not help us in any way comprehend the French language. We would be better served if we were just given the expression with its english correlation and told to memorize them. Free or not, this is not the best way to get someone to learn.
Everyone learns differently, so for you to say it is not the best way to teach someone or get someone to learn simply based on yourself is a bit much.
Duolingo is just a tool, use it as you see fit, use it in whatever way works for you and in conjunction with whatever other tools work for you.