Thank you to all who responded. Based on some of the comments, I realize that I should have noted that other users and devices use the wi-fi network, so it should be available all the time. Sorry about that omission.
In case it wasn’t clear earlier, the “modem” (in quotation marks because I believe “bridge” would be more accurate, and I welcome corrections) from the ISP has two ethernet ports. One ethernet port provids a signal to an Airport Express, which provids wi-fi to the residence. I was trying to connect the other ethernet port to my MacBook (through adapters, of course).
In retrospect, it appears that my problem was caused by the Airport Express router mode (in the Network pane of Airport Utility) being set to “Off (Bridge Mode)” rather than DHCP. Once I switched it to DHCP and NAT, the second ethernet connection from the ISP’s device started providing internet access to the MacBook. (Why did I have it set to Bridge Mode? Some hotel required that setting for it to work. Sometimes, the Airport Express travels with me.)
Now, while I’m in my office, I can use ethernet for internet access. While I’m elsewhere in the residence, I will use wi-fi, as I did before. Other devices will use wi-fi for internet access. Another Mac and the MacBook (when it is in my office) will use the Time Capsule for Time Machine and printer access—I hope. I told Time Machine to Back Up Now, and it started very slowly, even by Time Machine standards. But it did, apparently, back up. And the printer works.
I don’t personally care how long a backup takes to complete. It doesn’t slow down my computing experience in any perceptible manner the way waiting for an Internet download or web page rendering to take place does. Perhaps my experience is different from Time Capsule users as my TM backups go to dual USB connected hard drives.
Correct, I did get internet via wi-fi. I wanted to get internet via ethernet, because my ISP said it would be better, faster, more reliable, help me lose weight, lower my tax burden, keep my hosiery from running—wait, just the first three. But seriously, the ISP was adamant that using my Airport Express was hurting my browsing experience, and all would be rosy if I just used ethernet. Now that I got internet over ethernet, I am convinced the ISP was lying, probably unknowingly.
I still need the wi-fi for iOS devices, another Mac, and my MacBook when I use it in the living room.
As you discovered your ISP was lying. Even WiFi 802.11 G is faster than most internet coming into the home, WiFi is usually not the bottleneck in using the internet at home. Even now with my 200Mbps cable, I found it was not my 802.11 n WiFi but the Cat 5 wired router. (I don’t have WiFi enabled on my router that is in a far off corner of my house were the cable comes in, and Cat 5 wire to WiFi access points more centrally located.
As I mentioned earlier, I did get the MacBook to use two ethernet connections at the same time. Thanks again to all who helped.
According to the Network pane in System Preferences, I still have two ethernet connections (green indicators), one for internet (connected to the ISP’s device) and one for Time Machine (connected to a Time Capsule that does not have internet access). This is the setup that had been working. Now, however, Time Machine complains that it cannot find the backup disk, and the indicator on the ISP’s device for the ethernet connection to the MacBook blinks yellow rather than green. (The other ethernet connection on the ISP’s device, for the Airport Express, blinks green.)
Interestingly, I still get internet access over the ethernet from the ISP’s device, and Speed Test reports results close to what they had been. (As I said earlier, I believe the proper term for this device is a bridge. The ISP calls it a modem.) I switched ethernet ports on the Time Capsule and I swapped ethernet adapters on the cable from the Time Capsule to the USB hub; Time Machine always complained about the backup disk not being available. There is an iMac that has successfully backed up to the Time Capsule over ethernet since the MacBook’s problems began. I have restarted the MacBook a few times (not many, but more than once).
My conclusion is that something on the MacBook related to ethernet has died and the only reasonable cure is replacing it. Comments or suggestions?