Great question, Tammy. In general, we use the gerund as the subject of the verb “to be” and we use the infinitive as the predicate nominative of the verb “to be”. For example, we could use the gerund as the subject and say, “Having my own office has always been my dream”. Another example: Studying is fun. / It’s fun to study. This is not a strict rule of grammar, so in many cases it would be fine either way, but in the example in the OP, it sounds really weird to use the gerund in the predicate. For now, my advice would be for you to stick to the rule, but be aware that there is usually some flexibility when native English speakers are talking, and also, there are some fixed expressions and famous quotations that break the rule (or “guideline”, if you prefer).

It has always been a dream of mine to have my own office. Having my own office has always been a dream of mine. . Interesting. May need to ask someone skilled in grammar, such as a teacher. . The rule is not why it happens. The rule describes what happens and is accepted. Grammar describes the accepted constructs of the language, rather than dictates. . It may help learners, to have a rule that is easy to remember, and this may help constructing gramatical sentences, but it describes. . BTW Why can't I use the gerund in this case? <-- shifted the "I". :)

I believe your sentence is correct. This falls in the area of "style" and different native speakers will have different opinions. I personally would use the second sentence. But I don't think yours is wrong. Oddly enough, TO MY EAR, "Having my own office has always been a dream of mine" sounds natural, and "To have my own office has always been a dream of mine" sounds awkward.

Hi Tammy. I think your sentence is ok. I would happily use the gerund or the infinitive in this example. I would like to correct your question though - 'Why can't I?', not 'Why I can't?'