Ni muhimu kujifunika na blanketi usiku ili kupata usingizi mzuri.

It literally means “is important.” Swahili doesn’t need a dummy “it,” so you just say Ni muhimu + infinitive to mean “It’s important to ….” The ni is the copula “is/are” and is needed.

• Ni muhimu kunywa maji. = It’s important to drink water.
• Si muhimu kubeba mzigo wote leo. = It’s not important to carry the whole load today.

Swahili uses impersonal predicates without a dummy subject. Ni muhimu, Ni rahisi, Ni vigumu, Ni bora, etc., all work without “it.” You can add a beneficiary if you want:

• Ni muhimu kwa watoto kulala mapema. = It’s important for children to sleep early.

Not if you mean covering yourself. kufunika is “to cover (something/someone else).”

• Alimfunika mtoto kwa blanketi. = He covered the child with a blanket. For covering yourself, you need the reflexive: kujifunika.

All three occur:

• kujifunika na blanketi (with a blanket) — very common and natural.
• kujifunika kwa blanketi — treats the blanket as an instrument; slightly more formal.
• kujifunika blanketi — also heard (no preposition). None is wrong; prefer na or kwa in careful speech/writing.

The common plural is mablanketi (class 5/6 pattern):

• blanketi moja, mablanketi mawili. You may also hear some speakers use invariable blanketi for both singular and plural in casual speech, but mablanketi is the safe, standard plural.

Time-of-day words often act adverbially with no preposition: asubuhi (in the morning), mchana (in the afternoon), jioni (in the evening), usiku (at night).

• Nitaondoka asubuhi.
• Tulifika usiku. You can expand it when needed: usiku wa baridi (on a cold night), usiku kucha (all night long).

Yes, to change emphasis:

• Usiku ni muhimu kujifunika na blanketi ili kupata usingizi mzuri. (Topic: at night)
• Ni muhimu usiku kujifunika na blanketi ili kupata usingizi mzuri. (Inserts the time focus before the verb phrase)

ili introduces purpose: “in order to/so that.” Two common patterns:

• Same/unspecified subject: ili + infinitive (as in the sentence).
• Explicit subject: ili + subjunctive with a subject marker. Examples:
• Ni muhimu kujifunika … ili kupata usingizi mzuri. (generic)
• Ni muhimu kujifunika … ili upate usingizi mzuri. (so that you get …)
• Ili nipate usingizi mzuri, najizima simu mapema. (so that I may get …)

Close, but with a nuance:

• kupata usingizi = to get (some) sleep, to fall asleep / obtain sleep.
• kulala vizuri = to sleep well (quality of the sleep itself). Both are fine here:
• Ni muhimu kujifunika usiku ili kulala vizuri. (equally natural)

usingizi (sleep) is a mass noun. Common adjectives include:

• usingizi mzuri (good sleep), usingizi mzito (deep/heavy sleep), usingizi mrefu (long sleep), usingizi hafifu (light sleep). You’ll also hear mwema in set phrases (e.g., “Usiku mwema”), though mzuri is very common with usingizi.

Adjectives agree with noun classes. usingizi is an “u-” noun (class 11/14). The adjective “-zuri” takes the “m-” agreement here: m-zuri → mzuri. Compare:

• mtoto mzuri (class 1)
• kitabu kizuri (class 7)
• nyumba nzuri (class 9/10)
• usingizi mzuri (class 11/14)

Several options:

• Make the main statement negative: Si muhimu kujifunika na blanketi usiku. (It’s not important …)
• Negate the purpose with ili
  • negative subjunctive: … ili usipate baridi. (so that you don’t get cold)
• Express “without”: bila blanketi or bila kujifunika. (without a blanket / without covering yourself)

Use the imperative of the reflexive:

• Singular: Jifunike na blanketi usiku ili upate usingizi mzuri.
• Plural/polite: Jifunikeni na blanketi usiku ili mpate usingizi mzuri.

Yes:

• Ni bora … = It’s better to …
• Ni vyema … = It’s good/proper to …
• Afadhali … = It’s preferable/better … All can introduce similar advice with different strength of recommendation.