Learn English – Limitation, restriction vs constraint

Client and customer are no longer distinguished in the way they once were.

As recently as fifty years ago, client was still by and large reserved for people who sought advice from ‘professionals’ such as attorneys and architects. At that time client still reflected, even if only very dimly, the original Latin sense of the word as “one who is under the protection of” a patron. A client was someone who benefited from the professional’s learning and knowledge, while a customer was someone who had a purely commercial relationship with a tradesperson.

In this context, the term client acquired an honorific character. It flattered the person rendering services because it imputed a ‘professional’ degree of expertise; it also flattered the person who purchased the services because it imputed a degree of wealth and status—a person who routinely consults professionals is clearly someone above the common run of customers. And it happily set both at some distance from the vulgarity of commerce.

In consequence, from the last half of the 19th century, businesses of all sorts started referring to their customers as clients—‘exclusive’ tailors and hairdressers, grocers, insurance agents, even bookmakers (now calling themselves ‘turf accountants’). And since it is mostly businesspeople who talk about the relationship between buyer and seller, that use has gradually extended into ordinary speech.

The upshot is that today there is no difference whatever between customers and clients—they are the same people. Instead, customer is used when you wish to focus on the financial aspect of the relationship, client when you wish to focus on the personal service rendered.

to ponder

means to, over a period of time, think about the consequences of two or more choices in a decision. If there is only one thing you are pondering, you are pondering whether or not to do that thing.

to mull over

is the same as ponder - but you can mull over collective nouns, meaning you are trying to decide which one of the group. "I mulled over the candy, and found a good piece."

to contemplate

means to think about the consequences of something carefully, thoroughly, and for a long time. Typically when you contemplate something, you are deciding whether or not to do it, not picking among an array of choices. It has a serious connotation, likely due to the common phrase contemplate suicide.

to cogitate

I don't hear this word a lot. According to Google, it can be used intransitively, so you don't have to be cogitating about anything in particular. The other words here generally require a "target" to make sense.

to deliberate [on]

means the same as contemplate. When I hear this word, I think of court proceedings or meetings ("the jury will deliberate... etc.").

to ruminate

I don't hear this in speech very much. In my opinion it's a "less serious" version of contemplate.

to dwell on

means to think about something over and over. It could be the consequences of a decision not yet made, a decision that just has been made, or it could be going over past events in your mind. Dwell on can have a negative connotation, possibly that what you are dwelling on is bothering you or haunting you.

So, I would answer like this:

("depth 10k worths" doesn't make sense to me, so the second is a guess)

I have been dwelling on my son (all day) since he left home for college.

I have been mulling over my depth 10k worths because I will have to pay it by next week but I don't have enough money. So I have to find a way.

I have been dwelling on my new school project. I don't know where to start.

The government deliberated how to stop protest on raising student fees.