Learn Hardcore Swahili: Asubuhi kulikuwa na ukungu mzito, kwa hiyo tulitembea polepole. - There was heavy fog in the morning, so we walked slowly.

kuna (there is/are, present)
kulikuwa (there was/were, past)
kutakuwa (there will be, future) The na introduces what existed: kulikuwa na ukungu mzito = there was thick fog.

Could I say kuna ukungu mzito or kutakuwa na ukungu instead? When do I use each?

Yes:

kuna ukungu mzito = there is thick fog (now, present)
kulikuwa na ukungu mzito = there was thick fog (past)
kutakuwa na ukungu mzito = there will be thick fog (future) You’ll also hear kulikuwepo/kuna/kutakuwepo with -po to emphasize presence.

Why is the adjective mzito used with ukungu, not nzito or something with u-?

Adjectives agree with noun class, not with the surface prefix on the noun. Ukungu is class 11 (u-), and class 11 takes the adjective prefix m- for many adjectives, so mzito is correct. Nzito is the class 9/10 form. Also, uzito is a noun (weight), not an adjective.

What noun class is ukungu, and does it have a plural?

Ukungu is class 11 (u-). It’s a mass noun (fog), so it generally has no plural. If you needed a plural idea, you’d typically rephrase (a lot of fog, patches of fog) rather than make a formal plural.

Why is there no preposition before asubuhi? In English we say in the morning.

Time words like asubuhi (morning), mchana (afternoon), jioni (evening), usiku (night) act as time adverbials on their own. No preposition is needed: Asubuhi kulikuwa... = In the morning there was...

Where else can I put asubuhi in the sentence?

Flexible options:

Asubuhi kulikuwa na ukungu mzito, kwa hiyo tulitembea polepole. (time frame first)
Kulikuwa na ukungu mzito asubuhi, kwa hiyo tulitembea polepole. (time at the end of the first clause)
Kulikuwa na ukungu mzito, asubuhi kwa hiyo tulitembea polepole. (less common; better keep it with the first clause)

Can I say asubuhi hii or jana asubuhi to be more specific?

Yes:

asubuhi hii or asubuhi ya leo = this morning
jana asubuhi = yesterday morning Placement is the same as asubuhi in the original sentence.

What is the difference between kwa hiyo, kwa hivyo, and kwa sababu?

kwa hiyo = so/therefore (result). Very common as a sentence connector.
kwa hivyo = in that way/so; often interchangeable with kwa hiyo, but slightly more manner-like.
kwa sababu = because (introduces the reason). To say for that reason specifically, use kwa sababu hiyo.

Is the comma before kwa hiyo required?

It’s stylistic. You can write:

Kulikuwa na ukungu mzito, kwa hiyo tulitembea polepole.
Kulikuwa na ukungu mzito. Kwa hiyo tulitembea polepole. Both are fine.

Could I use the narrative -ka- tense after kwa hiyo?

Yes. Kwa hiyo tukatembea polepole means so then we walked slowly, highlighting sequence. Tulitembea is a plain past; tukatembea adds a sense of then/and so next.

How is tulitembea built?

tu- (1st person plural subject = we)
-li- (past tense)
tembea (walk) So tulitembea = we walked.

Is polepole always the same form? Can it be split?

It’s an invariable adverb meaning slowly. You’ll see both polepole and pole pole in writing; both are acceptable. It does not take agreement or change with number/person, and you don’t use a preposition like kwa before it here.

Can I use taratibu instead of polepole?

Often yes. taratibu can mean slowly, gently, or carefully. polepole focuses on speed; taratibu adds a nuance of carefulness.

Why not say ilikuwa na ukungu mzito? What about palikuwa na?

ilikuwa na ukungu would mean it had fog, but you’d need a clear singular subject (it) already established, which you don’t have here.
palikuwa na uses the locative subject pa- and points to a specific place: Pale palikuwa na ukungu mzito (there, there was thick fog). kulikuwa na is the neutral existential there was/were.

Could I say tulienda polepole instead of tulitembea polepole?

You can, but kuenda/kwenda is go, not necessarily on foot. Kutembea specifically means walk. If the idea is walking, tulitembea polepole is the most precise. If you just mean we moved/went slowly (by any means), tulienda polepole works.