ἐνθυμέομαι

Matthew 1:20 ἐνθυμηθέντος was pondering

But after he had pondered these things, an angel of the Lord appeared to him in a dream and said, “Joseph, son of David, do not be afraid to embrace Mary as your wife, for the One conceived in her is from the Holy Spirit.

Here: To turn something over in the mind and form an intention about it. The past-tense form most naturally means he had already thought it through and settled on his plan (v. 19) when the dream came, though 'while he was considering' is also possible.

Matthew 9:4 ἐνθυμεῖσθε think

But Jesus knew what they were thinking and said, “Why do you harbor evil in your hearts?

Here: Present tense: "why are you pondering…". It describes an ongoing process of reasoning, not a passing flicker. It has the same root as the noun "thoughts" earlier in the verse, so Jesus throws their own inner activity back at them.