Introducing は "wa" and です。"desu." the particle は marks the topic. (Remember particles attach to the word which they follow, and that when は is a particle it is pronounced wa.) です。functions like "is" or "topic is."
Is pronounced as "wah". は a subject or topic marker. は can appear as ha, or wa in romaji depending on the system used.
The kana used for は is usually pronounced as "hah", but pronounced as "wah" when used as a particle. The same phenomenon happens in the word "dewa/deha" では in the negative conjugation of the verb [desu].
Other notes: In yakuza speech, は、becomes や (yah). Typically, the y sound is softened such that it sounds like "ah" Example: Ore ha --turns to-> Ore ya --but sounds like-> Ore a
In [ex #7818], shouldnt it be: 皆さんはからどこですか? Or から皆さんはどこですか？ Wouldnt ex #7818 just mean "Where is everyone"? Im no Japanese grammar expert but doesnt から mean from? And shouldnt から be used in ex #7818?
See here and here for two very interesting Japanese pages on using ha vs. ga
When to use は vs が? Both sentences like 雨は降っている。[ame ha futteiru] and 雨が降っている。[ame ga futteiru] are valid, so working out when to use which form is a tricky point. First: In answering a question such as Q. 机の上に何がありますか。[tsukue no ue ni nani ga arimasu ka] A. 鉛筆があります。[enpitsu ga arimasu]. Here が is used.
On the other hand if attention has been placed on something in advance and the question is asked with that object as the topic... Q. 本はどこにありますか。[hon ha doko ni arimasu ka.] A. 本はイスの下にあります。[hon ha isu no shita ni arimasu] then は is used.
が is also used to indicate a specific choice. 私が行きます。[watashi ga ikimasu] _I'll_ go. (not any of the other possibilities).
は is used for contrastative sentences (see ha-2). For example normally you'd say 犬が好きです。[inu ga suki desu] I like dogs. However if you want to contrast how you feel about dogs with how you feel about other animals then 犬は好きだが、猫はどうも・・・[inu ha suki da ga, neko ha doumo ...] I like _dogs_ but I really don't (get on with) cats.
は [ha / wa] follows a noun or a noun phrase. (e.g. 犬は [inu ha] 'as for dogs' 知るのは [shiru no ha] 'as for what (I) know' The latter case の nominalizes (turns into a noun) 知る) N = noun