Nilikuwa nikifikiria kukupigia simu, lakini ukaja kwangu ghafla.

Literally, nilikuwa nikifikiria is:

• ni- = I
• -li-kuwa = was (from kuwa, to be)
• ni- = I (again, subject prefix)
• -ki-fikiria = thinking

So word-for-word it’s like saying: “I was I-thinking”, i.e. “I was thinking”.

Swahili often uses kuwa + another verb with -ki- to express a past continuous or ongoing action in the past.
So nilikuwa nikifikiria = I was thinking / I had been thinking.

• nilikuwa nikifikiria

  • Past continuous or ongoing: “I was thinking (for some time / at that moment).”
  • Suggests a process, not just a single mental event.

• nilifikiria

  • Simple (completed) past: “I thought.”
  • Sounds more like a single, complete act of thinking.

In this sentence, nilikuwa nikifikiria matches the idea that you were in the middle of thinking about calling, when something else happened.

They are very close, and both are understood as “I was thinking”:

• nilikuwa nikifikiria

  • More clearly marked as past continuous, using -ki-.
  • Feels a bit more “standard textbook” for that meaning.

• nilikuwa nafikiria

  • Uses na-, which normally marks present progressive (I am thinking), but with nilikuwa it gets shifted into the past: I was thinking.
  • Also common in speech.

In most everyday contexts, both would sound fine here.

The -ki- in nikifikiria marks an action as ongoing, habitual, or happening at the same time as something else.

In this structure:

• nilikuwa nikifikiria
  
  • nilikuwa = I was
  • nikifikiria = while I was thinking / in the process of thinking

So the -ki- form here helps express the idea of a continuous process: I was (in the middle of) thinking…

Breakdown of kukupigia simu:

• ku- (first) = infinitive marker, to…
• -ku- (second) = object prefix for you (singular)
• -pig- = root hit, beat, knock, dial
• -i- / -ia = applied extension “to/for”
• simu = phone

So ku-ku-pig-ia simu is literally:
“to you-hit-for phone”, i.e. “to call you (on the phone)”.

The two ku’s are doing different jobs:

• First ku- = “to” (infinitive)
• Second -ku- = “you” (object)

You keep both in forms like this; you don’t drop the infinitive ku- just because there is an object prefix.

You can say kupigia wewe simu, and people will understand:

• kupigia wewe simu = to call you (with wewe emphasized)
• kukupigia simu = to call you (with ku- as the normal object marker)

In natural Swahili, the object marker inside the verb (-ku- in kukupigia) is the default, neutral way.
You usually add wewe for extra emphasis, e.g.:

• Nilikuwa nikifikiria kukupigia wewe simu.
  = I was thinking of calling you (in particular).

In Swahili, object markers and verb extensions often carry information that English puts into separate words:

• kupigia simu is like “to call by phone” (lit. to hit-for phone).
• -pig-ia already contains the idea “to … for/with/to someone/something”.
• simu is the noun completing that idea: “phone”.

The typical pattern is: [verb phrase] + [noun]:

• kupigia simu = to call (by) phone
• kuandika barua = to write a letter

So simu naturally comes after the verb phrase kukupigia.

• ukaja = u- (you) + -ka- (narrative/sequence) + -ja (come)
• ulikuja = u- (you) + -li- (past) + -kuja (come)

The -ka- tense/aspect is often used to show a sequence of events or a sudden next action in a story:

• …nilikuwa nikifikiria kukupigia simu, lakini ukaja kwangu ghafla.
  = “…I was thinking of calling you, but then you came to me suddenly.”

So ukaja here feels like “and then you came” / “but you suddenly came”, more dynamic than plain ulikuja.

Yes, you can say:

• lakini ulikuja kwangu ghafla

It’s correct and means essentially the same.
The difference is nuance:

• ukaja with -ka-

  • Feels more story-like, sequential, sudden: “…but then you (suddenly) came.”

• ulikuja with -li-

  • More plain past, less emphasis on the “and then / suddenly” sequence.

In this particular sentence, ukaja … ghafla nicely reinforces the idea of suddenness.

kwangu means “to me / at my place / at my home / with me”, depending on context.

It’s formed from:

• kwa = at, to, with (a general locative preposition)
• -angu = my

Together kwa + angu → kwangu = “to/at my (place)”.

Other forms follow the same pattern:

• kwako = to/at your (place)
• kwake = to/at his/her place
• kwetu = to/at our place
• kwenu = to/at your (pl.) place
• kwao = to/at their place

ghafla means “suddenly / all of a sudden”.

In this sentence:

• …ukaja kwangu ghafla. = “…you came to me suddenly.”

Common positions:

• At the end: Ulikuja kwangu ghafla.
• Before the verb (for emphasis): Ghafla ulikuja kwangu.

End position, like in the example, is very natural and common.

lakini is the standard, most common word for “but/however”.

Alternatives include:

• ila – also “but/except”, often slightly more casual or contrastive.
  
  • Nilikuwa nikifikiria kukupigia simu, ila ukaja kwangu ghafla.

In most formal or neutral contexts, lakini is the default choice.

The sentence is very natural as it is. A native speaker could easily say it exactly that way.

A slightly simpler but still natural version might be:

• Nilikuwa nafikiria kukupigia simu, lakini ukaja kwangu ghafla.

Both versions are idiomatic; the original just leans a bit more into the ongoing past action with nikifikiria.