The difference between kwa sababu, kwa manaa, and kwa ajili.
I live in Tanzania. I use all these words on a regular basis, and I've asked a friend to help clarify because I rarely hear “kwa maana." I usually hear maana used in other ways.
This is long:
SABABU IS REASON. Kwa sababu is “why,” “because”, or “for (what/the) reason” depending on context. “Kwa sababu gani unaenda Dar es Salaam” means “For what reason/ Why are you going to Dar es Salaam?” “Kwa sababu nahitaji kuenda ofisi za uhamiaji” means “Because I need to go to the Immigration office(s).” You can also use the word “sababu, without “kwa.” For example, “Sababu kuu ya kufua nguo ni kuondoa uchafu.” This means, “The main reason to wash/launder clothes is to remove the uncleanliness/dirt.”
MAANA is MEANING. It is a word with many meanings (pun intended) and uses. I often hear it used as a noun, “Maana yake ni nini?” “what is its meaning?” “unajua maana yake?” “Do you know what it means?” Or, most often as a verb “kumaanisha” “to mean”. For example, if someone uses a word you don't understand, you can ask them, “Je, neno ‘Safari’ lina maana gani kwa kiingereza” “What is the meaning of the word ‘safari’ in English?” (litterally, "question-sound, the word safari has meaning what in English?") The person might respond, “Linamaanisha ‘long journey’” It means long journey.
I often use it to say “because”, like just now I said “tafadhali usizime fridge. Maana, chakula itaharibu.” my grammar might not be accurate but the person understood “please don't turn off the fridge. Because/meaning, the food will spoil.”
Maana is also used to ponder issues (mostly ideas and odd actions) one doesn't understand, or used as an alternative way to ask “why”, much like the way we use meaning in English.
Used at the beginning of a sentence it can have this use : “Kwa maana gani niko hai” meaning, “what is the meaning that I am alive.” better interpreted, “what it's the meaning of my life?”
Used in the middle of a statement: “Amenifanya kosa, kwa maana hiyo sitamwamini tena” this example might be inaccurate but it points in the right direction, it means “S/He did me wrong, so I'll never believe (in) her/him again”.
One can also use it to ask why someone did something odd or seemingly passive aggressive, (like your neighbor dirtied your freshly washed laundry and said nothing) but I am not skilled in Swahili enough to give an example.
If you come home to find your gate marked with a big red x, you might ask what it means. “kwa maana gani mtu ameandika x kwa mlango yangu?” (mlango yangu may be grammatically incorrect) Why did a person write an x on my door? Or “X hii(huu,hiyo?) ina maana gani? This x has what meaning? It likely means the gate was built in the road reserve area and will soon be demolished to build or expand a road.
Kwa ajili is much easier to use. It's like “on behalf of”. “Nilienda duka la dawa kwa ajili ya mama yangu. Anaskia maumivo. Nilimnunulia paracetamol.” I went to the pharmacy for (on behalf of) my mom. She's feeling pain. I bought her paracetamol(also known as acetomenephen/tylenol)” Or, the reason seemingly was given by someone else. Kwa ajili baba yangu alikua na asira, nimeondoka nyumba yetu kidogo, mpaka anatulia. “Because my dad was angry, I have left our house for a little while, until he calms down.”
Another good example. “Hii chakula ni kwa ajili ya nani?” Who is this food for?
Thanks for asking! I had never thought about how similar yet different these words are. I've learned something new.
My post above was meant to be a direct reply to this question you asked. I hope it helps.