You can. I was taught and have read that progressive tenses are to be used only when there a reason to point out that the action is being taken at a specific time, and that learners tend to overuse them.
I get that you are saying "cosa is feminine" so shouldn't you use "la" as the feminine pronoun to replace "thing" IMHO this is an overly narrow view of what we are trying to do here. The Spanish sentences should reflect what is most often said in Spanish, and the English sentences should reflect what is said in English. We are not looking for literal translations. The reason LO is used in Spanish is not because the speaker is replacing something masculine. LO is used as a neuter sort of wildcard pronoun which frequently substitutes for an entire concept or something said previously.
I disagree. It may not be something you hear often, but it is grammatically correct.
You would say "I used to say the same" The "to be" is redundant and sounds totally awkward, its not considered grammatically correct. When have you ever heard "I used to be" + gerund in English?
thekatmorgan, you are confusing common expressions with grammatically correct language. Many of the exercises given do not translate well into commonly-used English, but the translation is nonetheless valid. The point is not whether you can nitpick my expressions, but whether the translation is valid. "Ojos que no ven, corazón que no siente" translates into "Eyes that do not see, heart that does not feel", but the common expression in English is "Out of sight, out of mind". However, both translations are correct.
You are confusing idioms such as the example you gave with grammatically correct language. Idioms have to be adapted to the target language just as regular sentences do. When translators translate they do not do it literally word for word it has to be translated into grammatically correct English "I used to be saying the same" is not grammatically correct English "Yo estaba diciendo lo mismo" is "I was saying the same" the presence of the gerund here makes it "I was" rather than "used to" By your argument why are you not translating it as "I used to be saying it same" as that is the literally meaning or "Tengo sed" as "I have thirst" instead of "I am thirsty"
In English we never follow "I used to be" with a gerund. It has to be followed with a noun "I used to be a chef" an adjective "I used to be beautiful" or an adverb "I used to be madly in love with you."