In this phrase, is necessary the use of "I" twice?
I can do it by myself and I believe that will be no problem
I can do it by myself and believe that will be no problem
It looks like that changes the meaning
English Language Learners Stack Exchange is a question and answer site for speakers of other languages learning English. It only takes a minute to sign up.Sign up to join this community
You may grammatically omit a repeated subject. Here is an example: "These ... followed their mercenary calling and took their wages and are dead." We have one subject, "these" for three verbs, "followed," "took," and "are."
Thus, your two sentences are both grammatical. The problem is that the first verb in each sentence has a modal, "can." In your first sentence, it is quite clear that the first verb refers to your potential to "do it" and the second verb refers to your actual belief that there will be no problem. "Can" applies only to the first verb. The second sentence probably intends the same meaning and will most likely be interpreted that way, but it is slightly ambiguous. It could mean that you have the potential to "do it" and have the potential to persuade yourself that there will be no problem. Whether what is to be understood implicitly with respect to the second verb is "I" or "I can" is not clear. There is a different flavor to "I can believe" than to "I believe."
In short, omitting words may be grammatical in certain circumstances, but it may create ambiguity.