Because / so
The reason family: two clause linkers, two sentence openers and the reason-stacker.
-kara
polite desu kara / -masu karabecause; so
から ですから / ますから
Reason clause + kara + result; also ends a sentence as a bare excuse ('isogashii kara'). Nouns and na-adjectives take da kara.
isogashii kara, ikenai.
I'm busy, so I can't go.
忙しいから、行けない。
abunai desu kara, ki o tsukete kudasai ne.
It's dangerous, so please be careful.
危ないですから、気をつけてくださいね。
Reason comes first, then kara, then the result: the reverse of English 'because'. As a polite refusal desu kara sounds pointed; use node there ('isogashii node, ikemasen').
Learn alongside: -node
-node
since; so (softer)
ので
Same reason-then-result order as kara, but sounds objective and softer; the standard choice for explanations and excuses to strangers. Nouns and na-adjectives take na node.
ashita hayai node, mou neru ne.
I've got an early start tomorrow, so I'm heading to bed.
明日早いので、もう寝るね。
isogashii node, ikemasen.
I'm busy, so I can't go.
忙しいので、行けません。
With friends kara is the default; node, when used, often clips to -n de ('isogashii n de'). In polite speech node still follows the plain form; desu node / -masu node is business-formal.
dakara
polite nanodeso; that's why
だから なので
Starts a sentence that states the result of what came before; said sharply ('dakara!') it means 'like I said'.
isogashii n da. dakara, ikenai.
I'm busy. So I can't go.
忙しいんだ。だから、行けない。
isogashii n desu. nanode, ikemasen.
I'm busy. So I can't go.
忙しいんです。なので、行けません。
Points forward to the result; datte points back to the reason. In polite speech, desu kara at the start of a sentence can sound insistent ('as I said'); nanode is the softer opener.
datte
polite demo ... n desu yobecause; but (excuse)
だって でも…んですよ
Starts a sentence that justifies yourself or pushes back, usually answering 'why?' or a complaint; the reason follows, often ending in n da (yo / mon).
datte, isogashii n da yo.
But I'm busy!
だって、忙しいんだよ。
demo, isogashii n desu yo.
But I'm busy, you know.
でも、忙しいんですよ。
Sounds sulky to a coworker; there, open with demo and give the reason with ...n desu (yo). Don't confuse it with noun + datte 'even X' or hearsay '...n datte' ('I hear').
-shi
polite desu shi / -masu shiand what's more; since ... and all
し ですし / ますし
Stacks reasons ('yasui shi, oishii shi'); a single trailing -shi hints there are more, like 'and all'. Nouns and na-adjectives take da shi.
mou osoi shi, kaerou.
It's late and all, let's head home.
もう遅いし、帰ろう。
mou osoi desu shi, kaerimashou.
It's late and all, so let's head home.
もう遅いですし、帰りましょう。
A trailing -shi gives a reason softly ('mou osoi shi'); it isn't a neutral 'and' for listing facts, use -te for that.