1. I think we are here to talk truly.

2. I think we are here to talk truthfully.
3. I think we are here to talk t______

(Are they all grammatical and the same? What other adverbs can we use starting the letter t?

I wouldn't use 1, but 2 works fine.

You could also use these adverbs:

frankly
openly
honestly

Others can be found by looking up these three plus "truthfully" in a thesaurus.

1. "I think we are here to talk truly." - This sentence is grammatically correct. "Truly" is an adverb that modifies the verb "talk" and indicates that the speaker believes they are here to have a genuine and honest conversation.

2. "I think we are here to talk truthfully." - This sentence is also grammatically correct. "Truthfully" is another adverb that has a similar meaning to "truly" and emphasizes honesty and sincerity in the conversation.

3. "I think we are here to talk t______." - This sentence is grammatically incomplete because the blank is not filled. However, there are several adverbs that start with the letter "t" that can be used to complete this sentence, such as "thoughtfully," "tactfully," "timely," or "transparently." The choice of adverb will depend on the specific meaning or tone you want to convey in the sentence.