"The security guards began to eat sandwiches."
Translation:Ochroniarze zaczęli jeść kanapki.
The nasal sound ę is part of the past-stem of zacząć. It is featured in all forms that contain that stem, for example in zaczęty (passive adjectival participle), zaczęto (impersonal past), zaczęcie (verbal noun). It's just that ł and l have the property of denasalizing all nasal vowels.
With the rule that you propose you would have to watch out for ł/l so you that don't write an ę before them. With the rule in place, you'll have to watch out not to pronounce an ę before ł/l.
So both rules would be logical, with the second (real) one being preferable, since vowel alternations (especially denasalization) feels more intuitive in speech than in writing.
Just wondering why "ochroniarzy" is given as the first suggested translation in the pop-up when you hover over the words "security guards". That would be the accusative and/or genitive wouldn't it? Doesn't the popup usually suggest the translation in the correct case for the word in the context of the sentence at hand?
The 'beginning' is perfective here - there was a moment when they began, so now it is... begun ;)
But what they began is the process of eating. This is imperfective. If they began eating ten seconds ago, they definitely haven't finished. Even if they did, that's still imperfective because the main verb here is 'began'.
If you want to say that they finished eating, sure, that's perfective "Ochroniarze zjedli kanapki".