The Ashkenazic custom is to say “Ahava rabba…” in the morning and “Ahavas olam…” at night. (MB 2)
If need be, you can fulfil your obligation of krias shema by itself without saying the accompaning berachos. Afterwards you can say the berachos by themselves without krias shema. However, it is better to repeat krias shema with the berachos so you can stand up to say Shemoneh Esray from learning words of Torah. (ShA 2 – MB 4)
If you need to say Birkas Hamazon and you also want to help teach children how to bentch, either have in mind to fufil your obligation while teaching them, or have in mind not to fulfil and then bentch for yourself later. (BeH “lotzais”)
We pasken mitzvos need kavana to fulfil your obligation – even on mitzvos d’rabbanan. However, if you didn’t have kavana – although you must repeat the mitzva, don’t repeat the beracha before the mitzva. (ShA 4 – MB 10)
L’chatchila, you must have active kavana to fulfil your obligation before any mitzva. Bedieved, you don’t need to repeat the mitzva if circumstances around the mitzva show that you indeed want to perform the mitzva. (MB 10)
You must pay attention to what you are saying during krias shema. If you didn’t concentrate on the meaning of the first possuk and on “Boruch shem kavod…” you haven’t fulfilled your mitzva and you must read again with kavana. (ShA 5 – MB 11 – MB 63:12)