て isn’t “because”, it’s relational, in this usage anyways. your first sentence is essentially:
>being difficult, i didn’t understand it at all
it’s two separate phrases, side by side because they’re related, but not necessarily a direct explanation.
the second is
>because it’s difficult, i didn’t understand it at all
that one is a clear explanation. taking it further:
>むずかしかったですから**こそ**、ぜんぜんわかりませんでした。
becomes
>precisely BECAUSE it’s difficult, i didn’t understand at all
each puts successively more emphasis on the first phrase
you can also dig up nuanced differenced between から and ので, but that’s for another post (and it’s been posted a bajillion times, so search the feed)
て gives an implied reason. It’s not as strong as saying から or ので, which explicitly give the reason.
頭が痛くて、宿題をしませんでした。My head hurt and (therefore, as you should be able to gather,) I didn’t do the homework.
頭が痛かったから、宿題をしませんでした。Because my head hurt I didn’t do the homework. (That was the reason.)
たくさん食（た）べてお腹（なか）が痛（いた）いよ。
I have an upset stomach because I ate a lot.
I ate a lot and I have an upset stomach.
コーラを飲（の）んで寝（ね）られない。
I cannot sleep because I’ve drunk coke.
I’ve drunk coke and (so) I cannot sleep.
て IMPLIES the reason/cause In general when it is followed by verbs in potential form, and words expressing feelings て困（こま）る、て嬉（うれ）しい、て大変（たいへん）、てびっくり、て疲（つか）れている、て心配（しんぱい）、て休（やす）む、て安心（あんしん）、て◯が痛（いた）い、て気持（きも）ちがいい).
Note that native speakers NEVER say it in this way of speaking, “past form + **ですから**”. I know a lot of Japanese beginner-to-intermediate learners tend to make this mistake.
The second sentence should be: むずかしかった**ので**、ぜんぜんわかりませんでした。
A difference between the two is that the second is a formal or written form while the first is more casual though not limited to a verbal expression.
むずかしかった**から**ぜんぜんわからなくて…
The above is an example you can say it using “past form + から” though it’s limited to a verbal usage.
くて it more of an and (this and that happened) から is so or because (I did this so this happened)
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て isn’t “because”, it’s relational, in this usage anyways. your first sentence is essentially:
>being difficult, i didn’t understand it at all
it’s two separate phrases, side by side because they’re related, but not necessarily a direct explanation.
the second is
>because it’s difficult, i didn’t understand it at all
that one is a clear explanation. taking it further:
>むずかしかったですから**こそ**、ぜんぜんわかりませんでした。
becomes
>precisely BECAUSE it’s difficult, i didn’t understand at all
each puts successively more emphasis on the first phrase
you can also dig up nuanced differenced between から and ので, but that’s for another post (and it’s been posted a bajillion times, so search the feed)
て gives an implied reason. It’s not as strong as saying から or ので, which explicitly give the reason.
頭が痛くて、宿題をしませんでした。My head hurt and (therefore, as you should be able to gather,) I didn’t do the homework.
頭が痛かったから、宿題をしませんでした。Because my head hurt I didn’t do the homework. (That was the reason.)
たくさん食（た）べてお腹（なか）が痛（いた）いよ。
I have an upset stomach because I ate a lot.
I ate a lot and I have an upset stomach.
コーラを飲（の）んで寝（ね）られない。
I cannot sleep because I’ve drunk coke.
I’ve drunk coke and (so) I cannot sleep.
て IMPLIES the reason/cause In general when it is followed by verbs in potential form, and words expressing feelings て困（こま）る、て嬉（うれ）しい、て大変（たいへん）、てびっくり、て疲（つか）れている、て心配（しんぱい）、て休（やす）む、て安心（あんしん）、て◯が痛（いた）い、て気持（きも）ちがいい).
から is not limited in this way.
If you are curious you can read more about te form here:
https://www.reddit.com/r/LearnJapanese/comments/v08pbp/brief_japanese_history_of_て_form_why_is_it_so/
むずかしかった**ですから**、～
Note that native speakers NEVER say it in this way of speaking, “past form + **ですから**”. I know a lot of Japanese beginner-to-intermediate learners tend to make this mistake.
The second sentence should be: むずかしかった**ので**、ぜんぜんわかりませんでした。
A difference between the two is that the second is a formal or written form while the first is more casual though not limited to a verbal expression.
むずかしかった**から**ぜんぜんわからなくて…
The above is an example you can say it using “past form + から” though it’s limited to a verbal usage.
くて it more of an and (this and that happened)
から is so or because (I did this so this happened)