past or present?

[JY__]

"The white dog that died was being chased by another dog when it suddenly decides to dash across the road. I watched everything in horror as my heart twinged for what i'd just witnessed."
I'm actually confuse whether i should use dash, or dashed for the first sentence, and whether I should use "watch" and "twinge" instead for the second sentence. It seems right either way.
Can someone please help me out with this, and maybe explain how a sentence that is narrating a past event can have words that are in present tense.
Thanks
-someone who's trying to improve on the language-

[Casiopea]

"was" is past, so continue on with that tense:

"... was being chased by another dog when it suddenly decided to dash across the road. I watched everything in horror as my heart twinged for what I'd just witnessed."

Note, "I'd" is a contraction of I had (past tense).

About "dash", it's part of the to-infintive "to dash". Infinitive verbs do not carry tense.

Welcome.