The -ing phrases are participle phrases.
They are allowed at the end of sentences, but you want to be careful that you don't accidentally make a misplaced modifier:
*"Steve stood and watched a man, running his hands through is hair." This sentence is not good because it is hard to tell if Steve running his hands through his hair, or if the man Steve watches is running his hand through his hair.
You can use participle phrases as long as it is very clear which noun they modify.
“You seem forced” can be used to indicate what you’ve expressed
Your meaning may have been gotten better than you think. “You seem forced” employs a fairly common usage of “forced”, indicating strain even though there is not a physical force. Note that “you look forced” in your question’s title is a little different, having to do with the way that “seem” refers more comprehensively to apparent behavior and “look” refers more to physical appearance.
Another way to say this that would be specific to tone of voice, diction, etc. would be “you sound forced” which could be hedged a little as “you sound a bit forced” and would be helpfully followed by a bit of explanation or a question, such as “is everything OK?”¹ or “do you feel you have to talk to me?”
Expanding on the “look”/“seem” difference in “you don’t look yourself”
With “you don’t look yourself”, it seems like you are saying that something about the other person’s physical appearance is unusual compared with historical data or some assumed expectation (e.g. someone’s face is green or streaked with tears and you assume they are not always that way, even if you don’t know them).
If you were to say “you don’t seem yourself”, your comments would be understood to refer to someone’s overall presentation. This could certainly include physical appearance, but is more often used to refer to someone’s speech, body language, and other aspects of behavior. I believe this phrase would fit the situation you described.
Note for “you don’t seem yourself”: Similar to “you don’t look yourself”, you would be understood to either be making a comparison to ways you’ve seen this person act before or ways you assume they normally act. For example, you think that the person doesn’t normally put so much emphasis on their words or deliver them in a staccato rhythm.
The note from the end of §1 of this answer applies here as well. Since you’re guessing at something about another person, it probably wouldn’t be a bad idea to describe a little more what you mean by what you’re saying. I personally think it would be appropriate to ask a question aimed at learning more about how this person is feeling and/or what they think about the situation.
Best Answer
The full-clause form
You are correct: how about can also take a full clause with a subject of its own. A common sort of example is “How about we eat at Sparky’s Diner?” Judging by this graph, this phrasing only started becoming common in print around 1980, and in speech it might be only about 100 years old. The gerund form with an explicit subject (see below) is older and more formal, although that appears to be changing.
In How about subject verb {objects}?, the verb is in the subjunctive mood, but usually people put it in the indicative even though the meaning is subjunctive. Here are two examples in the subjunctive mood:
Notice that the negation is done in the usual way for the subjunctive: by putting not in front of the verb, without adding an auxiliary verb. In the indicative mood, these sentences would be:
Here, negation works as usual for the indicative mood.
The gerund form
The gerund form can also take a subject, usually in the possessive case:
This explains the missing “us” you noticed. Made explicit (as is almost never done), that would be:
Notice that negation works as usual for gerunds (that is, without an auxiliary verb).
The present-participle form
You can also say:
Notice that negation works as usual for participles (that is, without an auxiliary verb). You can even do this:
Why all this makes sense
The above all makes sense and doesn’t come across as ungrammatical, even in the rarer forms, because how about x simply raises x for consideration or to get an answer from the listener. The x can be anything: a physical object, a fact, or an unrealized possibility.
“How about this necklace?” asks for the listener’s thoughts about the necklace in regard to whatever the current topic of interest is. Depending on context, the meaning could be equivalent to “How would you like to wear this necklace?” or “How about I wear this necklace?” or “Do you think this is a remarkable necklace?” or even “Admit that this necklace would not have been found in your suitcase if you weren’t cheating on me!”
Raising an unrealized possibility or a fact for comment is really no different than raising an object for comment. The full clause or gerund clause still functions as a noun.
If it’s an unrealized possibility, like “How about you take out the trash?” or “How about taking out the trash?”, then it’s probably a suggestion or proposal.
Subjects are optional on gerunds, so both of these are grammatical: “How about taking out the trash?” and “How about your taking out the trash?”
You can also describe a possibility by naming an object and giving it an imagined adjective such as a present participle, as in: “How about you taking out the trash?” (As a strong hint to take out the trash, this form definitely comes across as informal and disrespectful.)
If it’s an actual fact, like “How about your missing all but two games last season?”, then you can’t use the full-clause form. That would contradict the subjunctive mood inherent in the full-clause form.
I can’t think of any other situations where a full clause can be the object of about. For example, you can’t say
“He’s talking about we hire John.”Maybe this is why you haven’t come across it in books or classes about English. (Most likely, books and classes skip it because it’s relatively new.)The only form that you really can’t use is the infinitive: