Yet, "je le pense" - like that - will barely be used for "I think so" - it would be really really formal, and sounds even a bit heavy.
"Je le pense" rather means "I mean it".
What is more often used, beside the seimple "je pense", is "Je pense bien" or "je pense que oui" as suggested here below.
Duo is suggesting that Oui, je pense is equivalent to Yes, I think so, in English, which is a pretty clear indication of agreement with previous conversation.
Yes, I am thinking could be an answer to ...Are you busy? ....Arjofocolovi is suggesting in his post that the French answer to that question would besomething other than Oui, je pense.
Such subtleties are beyond Google Translate's computer driven translations.
Some verbs are directly transitive (no preposition needed), like "appeler quelqu'un", some have direct and indirect constructions like "donner quelque chose à quelqu'un", some are exclusively constructed with one preposition, others with different prepositions for different meanings... and prepositions are not always "à" or "de". Some have "en", "sur", "dans", "par", "pour"...
So you are not dense, but French grammar is... Happy learning!
I would personally translate it more as "Oui, je réfléchis." Because the verb "penser" is more used to instant thinking or opinions, rather than active reflexion, or deep reflexion, which are often translated with "réfléchir".
But strictly speaking, it's not incorrect.
The answer is in the post just above.
Just to be clear : "Yes, I am thinking" can only be an answer to something like :
"Am I bothering / disturbing you ?" (Yes, "because" I am thinking)
"Are you looking for a solution / a way out / another option / etc. ?" (Yes, I am thinking, i.e. trying to figure out something)
- Are you thinking ? (Yes, I am [thinking, implied or not])
In the first two examples, the French answer will be "Je réfléchis" ; by default "to be thinking", i.e. "to ponder", "to reflect" is... réfléchir.
Now, as the third example may suggest, it can be the simple verb "to think", that you express in the present continuous. But then in French, to avoid the confusion with "Je pense [que oui]" (I think so), we use the structure that is the equivalent to present continuous, i.e. "être en train de + verbe" (which is usually not compulsory, but in some cases like this one, we'll definitely use it in order to be clear):
- Oui, je suis en train de penser.
Therefore, "Oui, je pense" cannot be "Yes I am thinking", at least by default and out of context.
OK, but when would you answer "Yes, I think" ?!?
you express that you're affirmative about something; then yoiu must say "I think so"
or you express what you are doing; then you must say "Yes, I am thinking" (as suggested above, answering a question like "Are you busy right now?"). In this case, the French equivalent would rather be "Je réfléchis / je suis en train de réfléchir".
The only possibility I can think of when you'd confirm that you think (in general, as the present simple implies), is answering a question like "Do you (sometimes / ever) think (about this, etc) ?". But then the answer should be "Yes I do".