Why is “的” used instead of “了” in this sentence?
To put it simply, the difference is:-
If you want to say -- &quot;Have they arrived already?&quot;, you use &quot;他和她一起来了吗?&quot; -- meaning you are just inquiring as to whether they have arrived, nothing more. The &quot;了&quot; here indicates &quot;already?&quot;, i.e. &quot;completion&quot;
But if you want to say -- &quot;Did they arrive together?&quot;, you use &quot;他和她一起来的吗?&quot; -- meaning you are inquiring as to whether they arrived together and not one-at-a-time or separately.
The &quot;的&quot; is for inquiring &quot;together?&quot; This is the reason for the suggestion of putting the &quot;是&quot; in there, which would make it to read &quot;&quot;Did they actually / really arrive together?&quot;, i.e. &quot;affirmation&quot;