I'm from the UK and I've also never heard this expression before... but as Greg has pointed out it seems to come from a phrase written by Mark Twain: “If it’s your job to eat a frog, it’s best to do it First Thing in the Morning. And if it’s your job to eat two frogs, it’s best to eat the BIGGEST one first.” It may be worth asking the person that originally told you the phrase to confirm, but from what is mentioned in the comments I think it's this quote. The phrase here means that you should prioritize things by doing the most unpleasant thing first. So if you have to do something difficult or unpleasant (like eat a frog) you should do it right away (first thing in the morning) and then the job is done (so you don't have to worry about it any more). You will probably find that a lot of people don't know this quote/phrase, so you won't be able to get the same meaning from an altered version. To make the most sense it would be best to say the quote in full if you were giving this as advice. If someone knows about the phrase, you could use a shortened version of it. For example if your friend needs to do their homework and they don't want to, you could say "eat the frog" to mean "do it now so you don't have to worry or think about it any more". But this would only work if the person you are speaking to knows the phrase. It could actually be fun to say "eat the frog" anyway, and see if they understand, then you could explain it if they don't.

"We will continue our lessons starting next week" is correct. "Since" would be used for a time in the past e.g: "I have been taking lessons since last week"

To put something/someone "on the shelf" is to say it is not needed/unwanted/abandoned (i.e. a book on the shelf is not being read). Haha, yes it does make you think of elf on the shelf! I feel like that toy is so well known the reference may be deliberate/a subtle joke.

If the purpose here is to explain that there is not enough time available to complete a task, I would say they are the same. Either could be used. If there is any difference, I think you would more often hear someone say "we don't have time" about a task happening right now e.g.: "We can't go to the store now, we don't have time" (before it closes) Often "we don't have the time" is about a task that doesn't have a set timeframe: "We can't learn to cook, we don't have the time" (we are always busy)

Great, thanks Ueda!

"Her endless shadows on your cheeks" is like a metaphor or artistic language. Here it can either be taken literally, or as an abstract idea. It is up to the reader to decide. I would think this means: The son has long eyelashes or defined cheekbones that cast a shadow, and these were inherited from his mother. OR A "shadow" on someone's face can be used to express emotion. This is often a melancholic (sad) emotion. This meaning is more abstract. You could say it means: The son's face shows similar emotions that his mother's face did. Blowing an eyelash is an old folklore/traditional thing people do for luck. When you lose an eyelash, you can hold it in the palm of your hand, blow it away, and make a wish. "There are days when I think someone must have blown all her eyelashes into the ocean just to wish for you." This means the father thinks that someone has made a lot of wishes to help the son. So the son is very lucky in some way.

"At" would sound the most correct in the answer to a question like this (UK English). "At the bank" (a banker would work in a bank, but "banker's" is not used for this kind of sentence) "At the shoe shop" "At the greengrocer's" Note - "greengrocer's" is correct because you are shortening "greengrocer's shop", this also works for "butcher's" (butcher's shop). "Banker's" would not be used because there is a separate word already which refer to that place (bank). "Baker's" to shorten "baker's shop" is sometimes used but usually people would say "bakery" (unless the bread is not made there). If you say "In the ~" it sounds like you mean something specifically inside the shop. For example if you were explaining the location of a particular place that was inside that building. EDIT - Sorry I just noticed you changed "banker's" to "baker's" which is correct. I'll leave my notes above though in case they are useful.