The את is because עגבניה is definite, has nothing to do with the verb. I agree, guard should be accepted here.
That's לְשַׁמֵר על yes? That's to preserve on it's own, keep on with the al. Reverso has it as reserve or maintain. Why do you keep insisting this is the answer? Is it the biblical Hebrew answer? Or are the translations wrong?
I agree it accepts "keep" or "save" so why not "guard"?
With so many crazy sentences on duo it's easy to imagine one of London's Beefeater guards protecting the queen's tomato.
It would greatly help to have a slow version of the audio. The speaker speaks much too rapidly for a beginner to grasp any meaning.
This is the course I started before Duolingo: http://www.memrise.com/course/1031737/hebrew-duolingo/
It's all the vocab used in Duolingo broken up the same way Duolingo does with audio. But no grammar. I'm on #41, I've found it very helpful to know the vocab before using it sentences.
Ok also in the Reverso app you can get the computer voice version of all the words in the dictionaries it has including Hebrew. You can slow it down in the settings. Mine is so slow she sounds drunk. (Note reverso website does NOT have the audio feature). (Btw, your last name is kind of like the Hebrew word for waiter I think... מֶלְצַר meltsar... I went to summer camp with Melzer brothers... )
TeribleTeri: I don't know why, but I can't get any audio in Hebrew on Reverso. I can get audio on each and every other language on their site (I believe), but not for Hebrew. Also, my Duolingo app doesn't usually allow me to use the turtle version of the audio for Hebrew, although it does for some other languages I'm studying and/or dabbling in. (But, hope someone else can use those pieces of advice.)