Uki Uki mini 7

Uki Uki mini 7
Q1 from derby boon: what's the difference between "ja arimasen" and "dewa arimasen"?
じゃ and では are pretty much interchangeable, except that では sounds more formal. Likewise, ありません and ないです mean the same, with ありません taking a slightly more formal tone. 
To inform someone "The meeting is at 4:00. It's not at 5:00," you say:
ミーティングは4時です。(Miitingu wa yoji desu.)
5時じゃないですよ。(Goji ja nai desuyo.) / 5時じゃありませんよ。(Goji ja arimasen-yo.) / 5時ではないですよ。(Goji dewa naidesuyo.) / 5時ではありませんよ。(Goji dewa arimasen-yo.)
All of these work well in this situation.
Q2 from Abdulrahman Majash: When is it ok to remove particles? 
The more casual your speech is, the shorter it usually is. In Japanese, a shortened speech often has certain particles omitted. But be careful! Not all particles can be dropped. The ones you can commonly drop are: は、が、and を. You can also drop に or へ as a destination particle.
今日はどこに行きますか？(Kyou wa doko ni ikimasuka?) Where are we going today?
→　今日どこ行く？(Kyou doko iku?)
私はカレーが大好きです。(Watashi wa karee ga daisuki desu.) I love curry!
→　私カレー大好き！(Watashi karee daisuki!)
よくどんな映画を見ますか？ (Yoku donna eiga o mimasuka?) What kind of movies do you usually watch?
→　よくどんな映画見る？(Yoku donna eiga miru?)
Try using casual Japanese without these particles. 
Q3 from Yeshwanth Bhatt: What's the difference between shiru and wakaru?
知る (shiru) implies that you have the information about the specific topic, whereas 分かる (wakaru) implies that you understand it. So 知る expresses knowledge or familiarity with something, whereas 分かる expresses comprehension. 
Let's imagine that a Japanese song you've heard before is playing in the office cafeteria. Your colleague asks you, "Do you know this song?" To say yes, you say "はい、知っています" (Hai, shitte imasu.) because your familiarity means that you have some information about it. If you've never heard it before, then you lack information, so you say "いいえ、知りません." (Iie, shirimasen.)  
If you want to tell your colleague that you recognize this song but unfortunately you don't really understand the lyrics, you can say, この曲は知っていますけど、よく分かりません。(Kono kyoku wa shitte imasu kedo, yoku wakarimasen.) 
To sum it up, 知る is about having some information or familiarity with something, and 分かる shows understanding on a deeper level.
Q4: aseel abdulaziz: "I want to learn Japanese. How can I start?"
First, welcome to Japanese!! ようこそ (Youkoso)! It's an extremely interesting, rich and beautiful language, and I hope you'll have tons of fun learning it. 
If you have no idea where to start, what I usually suggest is to find a textbook you like. If your city has a Japanese bookstore, spend a couple of hours in its Japanese language section browsing all the textbook options they offer. Pick one that you feel is the easiest to use, that you can visualize yourself using for a long time. If you don’t have a Japanese bookstore nearby, you can of course research online! Whichever the case, I recommend that you pick one book (not 5 or 6) that you really like and stick to it. Study the book thoroughly. 
Based on my experience of observing many students' reaction and progress, GENKI series and Minna no Nihongo are one of the books that I highly recommend. Perhaps you can start there see how it goes. I wish you the best of luck!!
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