This is a normal Alif and i really don't know why Duolingo is bothering people with it (we don't really use that mark on the top when we write normally). Anyway, it is called sometimes Hamzat-Wassl (connecting Hamza). It sounds like a regular Alif when in the beginning of the sentence or nothing preceding it, but when it is in the middle of a sentence with words preceding it, it kind of get eclipsed by the vowel before it (so, kind of acting like a schwa of some sort).
Using it: Well, for the time being, I believe it is just enough to know that it is used in the definite article "AL" الـ. There are some specific verbs and tenses that use this Alif and not أ but this is a bit of lengthy story.
If you like, you can search in the Arabic forum here on Duolingo for The Story of Hamza - I made a 3 parts posts about various types of Hamza (and accordingly, Alif at the beginning of words).
Take (IS) and divide the sentence into two parts: The left part is the subject, the initiator of the sentence, and the right side is the information, or the ending result.
Away the Arabic sentence for now, the English sentence above says: The window in the kitchen is broken - Which means the subject is (the window in the kitchen), let's ask ourselves: What about it? The answer is: it is broken.
Let's do the same to your sentence: The window is broken in the kitchen. The subject here which comes before (IS) is actually (the window) alone. What about the window? what window? which window? The answer comes after (is), and that is (is broken in the kitchen). In other words, your sentence states that the act of breakage happened in the kitchen. As if someone asked you (where is the window broken?). This of course is totally different from the original English sentence above which is telling a fact that there is a window in the kitchen and it is broken, as if someone is asking (what about the window in the kitchen?). As you can see, the difference is in the logic of the sentence, and what comes before (is) and after (is) makes a difference of what the sentence is really telling. As for the Arabic sentence, the translation is as stated above. Your answer would rather be translated as: الشباك مكسور في المطبخ or even الشباك كُسِرَ في المطبخ.
By meaning it is, but in syntax, it is not equal. Because your sentence uses the genitive between "kitchen" and "window" and that in Arabic would be شباك المطبخ
In the original sentence in Arabic, however, this is not the case. The sentence "locates" the window "in the kitchen" في المطبخ.
As I said, the meaning between the two is the same somewhat, yet the syntax and the structure is different, and I think Duo contributors here are looking for the structure rather than the core meaning of a sentence.
Actually it is Al-Matbakh but the wrong part in the audio is saying ت instead of ط in this word.
The Shaddah over consonants comes after AL only for specific letters (we call them Solar Letters حروف شمسية). They are:
د ز ط ل ن س ص ر ش ت ث ذ ض ظ
14 letters in total. Shaddah comes on those when they start the word and AL is put on the word to make it defined (i.e. THE). This is purely for phonetic necessities because the movement of the tongue from AL to, let's say, SH, would be cumbersome. Thus, the L is merged into SH and Shaddah is placed on it (i.e. the consonant is doubled). Example: šams (sun) -> aš-šams (the sun); shubbák (window) -> aš-šubbák (the window). In Arabic, the orthography does it not change except for adding Shaddah. This change or stress on the first consonant is also noticeable in Maltese (which for me is just another dialect of Arabic). Check Google and translate from English to Maltese and type (sun), then (the sun), you will notice it yields (xemx) for the first (x sounds as sh in English), and then (ix-xemx) for the second.