Nimepoteza kalamu yangu jana, kwa hivyo leo ninatumia peni ya kaka yangu.

Both nimepoteza and nilipoteza are grammatically possible; the choice is about nuance.

• nimepoteza = ni-me-poteza

  • ni- = I
  • -me- = perfect (completed with present relevance)
  • Rough meaning: I have lost / I have already lost
  • Emphasizes that the result is important now (I don’t have the pen today).

• nilipoteza = ni-li-poteza

  • -li- = simple past
  • Rough meaning: I lost (at some time in the past)
  • More neutral, just narrates a past event.

In this sentence, nimepoteza kalamu yangu jana, kwa hivyo leo... highlights that losing it yesterday explains the situation today. Using nilipoteza would still be correct; it just slightly reduces that feeling of “and that’s why I’m in this situation now.”

English is unusually strict about the present perfect; it normally blocks explicit past-time expressions like yesterday, last year, etc.

Swahili doesn’t follow that rule. The -me- form:

• Can appear with time words like jana (yesterday), leo (today), asubuhi (in the morning), etc.
• Focuses on a completed action whose result is still relevant.

So nimepoteza kalamu yangu jana is completely natural:

• “I have lost my pen yesterday (and the result affects me now).”

Swahili speakers use -me-+time-expression all the time; you don’t need to avoid it the way you do in English.

It’s not wrong; it’s fine.

• Nimepoteza kalamu yangu jana... – “I have lost my pen yesterday…” (with a stronger sense that the loss is relevant now).
• Nilipoteza kalamu yangu jana... – “I lost my pen yesterday…” (simple past; fits well in a narrative).

Both work in this sentence. Many speakers would choose nilipoteza here, especially in storytelling; others like nimepoteza for the “that’s why I’m using my brother’s pen today” feeling. It’s a stylistic choice, not a strict grammar rule.

ninatumia can cover both meanings; Swahili present -na- is broad:

• Right now / current situation:

  • Leo ninatumia peni ya kaka yangu.
    → “Today I’m using my brother’s pen.”

• Habitual / general truth:

  • Kazi yangu ni ofisini. Kila siku ninatumia kompyuta.
    → “My job is in an office. Every day I use a computer.”

Context and extra words give precision:

• Emphasize “right now”: add sasa or sasa hivi
  
  • Sasa ninatumia peni ya kaka yangu. – “Right now I’m using my brother’s pen.”
• Emphasize habit: add kila siku, mara nyingi, etc.
  
  • Kila siku ninatumia peni ya kaka yangu. – “Every day I use my brother’s pen.”

They are related but used in different ways:

• ninatumia = ni-na-tumia

  • Subject ni- (I) + present -na-
    • verb stem -tumia (use)
  • Full standard form for “I use / I am using.”

• natumia

  • Short/contracted form commonly used in speech and informal writing.
  • Meaning is the same as ninatumia.
  • In your sentence you could also say Leo natumia peni ya kaka yangu.

• kutumia

  • Infinitive/dictionary form: “to use.”
  • Used after another verb or preposition, not as the main finite verb:
    
    • Ninapenda kutumia kalamu. – “I like to use a pen.”
    • Nimeanza kutumia kompyuta mpya. – “I have started to use a new computer.”

In your sentence you need a finite verb, so ninatumia or natumia is correct, not kutumia on its own.

They overlap a lot; in everyday speech they’re often near-synonyms, but there are tendencies:

• kalamu

  • Older, from Arabic.
  • General “writing instrument”: pen, pencil, etc., especially in older or more formal usage.
  • You can specify:
    
    • kalamu ya wino – ink pen
    • kalamu ya risasi (rare) / penseli – pencil

• peni

  • Loan from English pen.
  • Usually specifically a pen (ballpoint, fountain pen, etc.), not a pencil.

So your sentence is like saying:
“I lost my writing instrument yesterday, so today I’m using my brother’s pen.”

Stylistically, many people would keep the same word in both places (either kalamu…kalamu or peni…peni), but mixing them is still understandable and grammatical.

In Swahili, possessive adjectives normally follow the noun:

• kalamu yangu – “my pen”
• kitabu chako – “your book”
• rafiki yake – “his/her friend”

So the natural order is:

[noun] + [possessive]

Literally, kalamu yangu is “pen my.”
Putting yangu before the noun (yangu kalamu) is ungrammatical in standard Swahili.

These look similar but they’re slightly different structures:

1. kalamu yangu – “my pen”

   • kalamu (pen) + yangu (my)
   • y- in yangu agrees with kalamu’s noun class, and -angu = “my.”

2. peni ya kaka yangu – literally “pen of my brother”

   • peni (pen)
   • ya (of – agreeing with peni)
   • kaka yangu (my brother)

So:

• With a pronoun owner (me, you, him…), you can use -angu, -ako, -ake, -etu, -enu, -ao directly after the noun:

  • kalamu yangu – my pen
  • kalamu yako – your pen

• With a noun owner (the brother, the teacher, John…), you must use the -a connector that agrees with the thing owned:

  • peni ya kaka yangu – pen of my brother
  • kalamu ya mwalimu – teacher’s pen
  • gari la mama – mother’s car

So yangu and ya are related forms, but:

• yangu = “of me” (my)
• ya = “of” before a noun possessor, agreeing with peni.

The -a connector (the “of” word) changes form depending on the class of the thing being possessed, not the owner.

• peni is in the same noun class as kalamu (often called class 9).
  
  • For that class, the -a form is ya.
  • So: peni ya kaka yangu – “pen of my brother.”

Compare with another noun:

• mtoto (child) is a different class (class 1).
  
  • Its -a form is wa.
  • mtoto wa kaka yangu – “child of my brother.”

So:

• peni ya kaka yangu (pen → class 9 → ya)
• mtoto wa kaka yangu (child → class 1 → wa)

The choice ya / wa / la / cha / vya / za / etc. always follows the thing that is owned.

There are two levels of possession here:

1. The pen belongs to the brother.
2. The brother belongs to me (my brother).

Swahili handles this like this:

• peni ya kaka yangu
  
  • peni ya kaka – pen of (the) brother
  • kaka yangu – my brother

The ya appears once, before the whole owner phrase (kaka yangu). Inside that phrase, kaka has its own possessive yangu (my).

You don’t stack ya again before yangu; kaka ya yangu is not used. One ya is enough to connect the pen to the entire phrase kaka yangu.

kwa hivyo is a connector meaning “so / therefore / as a result”. Literally, it’s like “by/with thus”.

In your sentence:

• ..., kwa hivyo leo ninatumia peni ya kaka yangu.
  → “..., so today I’m using my brother’s pen.”

Alternatives:

• kwa hiyo – very close in meaning; many speakers use kwa hiyo even more often.

  • Nimepoteza kalamu yangu jana, kwa hiyo leo ninatumia peni ya kaka yangu.

• basi – conversational “so / then / well then.”

  • Nimepoteza kalamu yangu jana, basi leo ninatumia peni ya kaka yangu.

Be careful not to confuse with kwa sababu (“because”):

• Ninatumia peni ya kaka yangu kwa sababu nimepoteza kalamu yangu jana.
  → “I’m using my brother’s pen because I lost my pen yesterday.”

So: kwa hivyo / kwa hiyo / basi = result; kwa sababu = reason.

They can move quite freely; moving them slightly changes emphasis but remains grammatical.

For the first clause:

• Nimepoteza kalamu yangu jana. – neutral, common.
• Jana nimepoteza kalamu yangu. – emphasizes “yesterday” a bit more.
• Kalamu yangu nimeipoteza jana. – emphasizes kalamu yangu (my pen) and then adds “yesterday.”

For the second clause:

• Kwa hivyo leo ninatumia peni ya kaka yangu. – neutral.
• Kwa hivyo ninatumia peni ya kaka yangu leo. – slight emphasis on “today.”
• You can also front it strongly in careful speech:
  
  • Leo ninatumia peni ya kaka yangu, kwa sababu nimepoteza kalamu yangu jana.

So jana and leo are flexible adverbs; their position mainly affects focus, not correctness.

In modern standard Swahili:

• kaka = male sibling, usually understood as “brother.”
  
  • Traditionally, it tends to imply an older brother, but in everyday usage many people use it for “brother” in general.
• dada = (female) sister, often older but widely used for any sister.

Some extra notes:

• Forms like kaka mkubwa (big/older brother) and kaka mdogo (younger brother) are also used in practice, even though kaka already leans toward “older.”
• Kaka can also be used more broadly as a respectful address for a young man (“brother,” “sir”).

In your sentence, kaka yangu is simply “my brother.”