Breakdown of mise wa mou aite imasu.
Questions & Answers about mise wa mou aite imasu.
は marks the topic: we’re talking “as for the store.” It assumes the store is already in the conversational background or is being contrasted with something else.
Using が—店がもう開いています—is also grammatical and presents “the store is already open” as new, noteworthy info you’ve just noticed or discovered. With は, you might be answering “What about the store?” or contrasting: 郵便局はまだですが、店はもう開いています。
It means “is open.” With many intransitive verbs like 開く, the V‑ている form describes the resultant state, not an ongoing action. So 開いています = “it has opened and is (now) open.”
If you truly mean “is in the middle of opening,” you’d describe the action or preparation: 今、開店準備をしています (“They’re getting ready to open”) or, focusing on someone’s action, 今、店を開けているところです (“They’re opening the store right now”).
Here います is the auxiliary that forms V‑ている (progressive/resultant state). This いる is used regardless of whether the subject is animate or not. The いる/ある split only applies to the standalone existence verbs.
Related pattern: V‑てある (with a transitive verb) describes a resultant state due to someone’s intentional action: ドアが開けてあります (“The door has been left open [by someone]”). For “is open,” use the intransitive: ドアが開いています.
