Wait, shouldn’t nghĩ be in the past tense? How’s the past tense formed?
I think past tense is coming up soon. In any case, yeah, this sentence should be "than I think" because there's no past tense here.
Sometimes in Vietnamese you can leave out words like đã, sẽ, or đang if the tense is implied. For example: Tôi cười which means I smile AND I am smiling.
More relevantly, take this sentence from my Vietnamese girlfriend who doesn't speak English: Em vừa mới từ tiệm đến đây, meaning I just came here from the shop. Note that she doesn't say đã đến.
Word placement is key. "I think this tunner is longer" = "Tôi nghĩ đường hầm này dài hơn". It's so subtle =|
I think this tunnel is longer than I think, because I have two personas.