だから details
Examples
so; that's why — the conclusion or decision that follows from what was just said
明日は朝五時に出るんだ。だから、もう寝るね。
ashita wa asa goji ni deru n da. dakara, mou neru ne.
I'm leaving at five tomorrow morning. So I'm going to bed already. casual
この道は工事中だ。だから、遠回りしたほうが早い。
kono michi wa koujichuu da. dakara, toomawari shita hou ga hayai.
This road is under construction. That's why going the long way round is faster. neutral
会場の収容人数には限りがございます。ですから、お早めにお申し込みください。
kaijou no shuuyou ninzuu ni wa kagiri ga gozaimasu. desukara, o hayame ni o moushikomi kudasai.
The venue's capacity is limited. For that reason, please apply early. formal
like I said — points back at the speaker's OWN previous utterance and restates it instead of drawing a new consequence
だから、その店は今日休みだって言ってるでしょ。
dakara, sono mise wa kyou yasumi datte itteru desho.
Like I said, that place is closed today. casual
Formation
だから、clauseattaches to plain clause
Notes
Formal writing reaches for そのため or したがって instead; ですから is the
polite face of the spoken word, not of the written one.
Related patterns
Similar
それで reports the result that actually followed and will not carry an intention announced on the spot or a command after it (それで、もう寝るね and それで、早く帰りなさい both fail); だから asserts the speaker's own reasoning and fronts exactly those — dropping だから into a plain account of events makes it sound as if you are arguing a point rather than telling what happened.
Lookalikes
One word boundary apart — inside a sentence 学生だから is the copula だ plus the reason particle から ("because he's a student"), while at the head of a sentence だから is a single connective meaning "so", and the reason it points back to is the previous sentence.
Both end in から and it is a different から each time — それから reads それ as a starting POINT IN TIME ("from there on"), while だから is the copula だ plus から the reason particle; それから、休んだ says the rest came next, だから、休んだ says it came as a consequence.