orangeacacia wrote:Can someone explain what exactly a subject phrase is?

Also, how do you identify it in a context?

Ex: Planting all these seeds is more involved than I thought.

My approach: I thought seeds was the subject so I assumed that it would be correct to say are, but the answer said that planting all these seeds = subject phrase.

Subject Phrase is nothing but the phrase which acts as a Subject.

In order to resolve the problem of finding the subject, you should always ask question for the verb.
For example,
In the above stated statement of yours.
Planting all these seeds is more involved than I thought.
You should ask, what is more involving.
And according to the answer you can clearly see that it is "planting" not "seeds" which is more involving.
Also, since planting is a gerund, it is always singular hence the verb is singular.

Some more examples for your reference :
Knowing the truth has helped them a lot.
Helping the poor has always been his main goal.
Ironing shirts is more complicated than washing them.

Hope it helps.
Thanks

Let 'Seeds' be the subject. Then the sentence is

Seeds is more involved than I thought. Now, let us correct the verb number.
Seeds are more involved than I thought. (Does the sentence mean something ??) No!

Another one.
Answering all the BTG questions is a great experience.

Answering is a great experience - Correct!
BTG questions is a great experience - Incorrect!

p.s: Anukrati explained the rest in a better way!