Mimi nitaenda posta kesho asubuhi.

Mimi is the first-person singular pronoun “I.” In Swahili verbs you already show the subject with a prefix (here ni-). You include Mimi for emphasis or clarity, but you can omit it:
• Nitaenda posta kesho asubuhi still means “I will go to the post office tomorrow morning.”

The verb nitaenda breaks down into three parts:
• ni- = subject prefix for “I”
• -ta- = tense marker for the simple future
• enda = the verb root “go”
Put together: ni-ta-enda = “I will go.”

The infix -ta- marks the simple future tense. You use it for actions that haven’t happened yet. Compare:
• Ninaenda = “I am going” (present)
• Nilienda = “I went” (past)
• Nitaenda = “I will go” (future)

Kwenda is a different verb root meaning “to go” in a general sense; it’s often used for “become” or “succeed in going.” When you mean “go” as movement, you use enda. So:
• nitaenda = I will go (to a place)
• nitakwenda would be awkward for simple movement

It’s a time phrase combining:
• kesho = tomorrow
• asubuhi = morning
Together they specify “tomorrow morning.” You could also say asubuhi ya kesho, but kesho asubuhi is more natural when you place it right after the verb.

Yes. Swahili is fairly flexible with time-place adverbials. You can say:
• Kesho asubuhi nitaenda posta
It still means “Tomorrow morning I will go to the post office.”

Absolutely. A common colloquial version is:
• Nitaenda posta kesho asubuhi.
You can also reorder:
• Kesho asubuhi nitaenda posta.
Either way, the meaning stays the same; Swahili relies on prefixes and context more than strict word order.