Постоянно means constantly / all the time and modifies пропадает. Russian word order is flexible; you might also see:
Because поэтому introduces a result clause (therefore/so). Here you have two parts with their own verbs:
Not exactly:
That’s the standard pattern: мочь + infinitive.
(я) не могу + отправить = I can’t send. The main verb is могу (I can), and отправить stays in the infinitive.
Отправить (perfective) focuses on completing the action—successfully sending the message. With не могу, Russian commonly uses perfective to mean can’t manage to do it (to completion).
Не могу отправлять would sound more like I can’t be sending (as an ongoing activity) or I’m not able to send (in general), but the perfective отправить is the natural choice for “send (successfully) this message.”
Yes. Russian often drops subject pronouns when it’s clear from the verb:
It can mean either, depending on context: in the metro/subway generally (stations, tunnels, trains). If you want to be more specific:
Key stresses: