て・
The て-form can give a reason. What follows is how you felt, something you couldn't do, or what came of it. The negative is なくて.
the negative
I was lonely because I couldn't see my friend.
couldn't
I forgot my key, so I couldn't get into the house.
How to form it
て-form of a verb or adjective + a feeling or a result
聞いて, 会えて, 忙しくて, 大変で. A request, an invitation or a suggestion can't follow: there the reason takes から or ので.
Negative
ない → なくて
会えない → 会えなくて.
How you felt, and why
Glad, relieved, surprised, disappointed, stuck: the て-form says what brought the feeling on. English often says "to" here: glad to see you, surprised to hear it.
I'm glad to see you again.
The "can" form, 会える: you are glad you were able to.
I was surprised when I saw the news.
It's a shame I can't come to the party.
The bus didn't come, and I was stuck.
Thank you for, sorry for
What you are grateful for or sorry about takes the て-form too, not から or ので.
Thanks for helping today.
It needs くれて: the helping was done for you.
I'm sorry I'm late.
What came of it
The て-form itself only says "and": one thing happened, and it led to the next. With "can't", English often says "too … to".
It's too dark to see anything.
The trains stopped, so I couldn't get home.
I couldn't hear the explanation clearly, so I didn't understand it.
I caught a cold and stayed home from school.
Not before a request or a suggestion
After a reason in the て-form comes something that is already so: a feeling, a fact, something that happened. When you ask, invite or suggest, or say what you are going to do, the reason takes から or ので.
I had no time, so I couldn't eat breakfast.
Something you couldn't do. The て-form fits.
Not said: 時間が
(We have no time, so let's hurry.)
A suggestion can't follow it.
We don't have much time, so let's hurry.
It can follow から.
Not said: 雨が
(It's raining, so please take an umbrella.)
雨が降っているから or 降っているので.
Sounds odd: 頭が
I have a headache, so I am leaving early today.
頭が痛いから or 痛いので. Set phrases like ちょっと用事があって、お先に失礼します are still heard.
なくて or ないで?
A verb has two negative て-forms. For a reason it is なくて. ないで is "without doing".
I didn't do my homework, and the teacher told me off.
Why the teacher was angry: なくて.
I went to school without doing my homework.
Not doing it isn't why you went. "Without": ないで.
Other jobs of て
The same form, joining actions in order with no reason in it.
I went home, did my homework, and went to bed.
The first verb says how the second is done.
I walk to the station.
Close neighbours
からN5
"Because" for any reason, and the one before a request, a suggestion or a plan.
It is dangerous, so please be careful.
のでN5
The softer "because". A request or a plan can follow it too.
I have a headache, so I am leaving early today.
Noun + でN5
When the cause is a thing: snow, a cold, an accident.
The trains stopped because of heavy snow.
The apology with すみません: for what you did, or with なくて for what you didn't do.
I'm sorry I didn't get in touch yesterday.
Check yourself
1Which one fits?
（ ）、
Choose an answer to see why.
2"Thank you for coming."
Choose an answer to see why.
More detail
Stopping at て
When people explain themselves, they often give the reason and stop. The rest is understood.
Sorry, I was busy.
More sentences
I was happy to hear my family's voices for the first time in a while.
I didn't know the way, and I started to worry.
It was a shame I couldn't get a reservation.
I lost my wallet, and I was stuck all day.
Work was hard, and I'm worn out.
I was too busy to come to the meeting yesterday.
The typhoon came closer, and the flight was canceled.