Вчера было сегодня
Sometimes a Russian sentence starts in the past tense and then suddenly switches to the present. For people learning the language this can look strange, because in English tenses usually match each other.
Russian works a little differently. What matters here is not strict tense agreement but the point of observation. Very often a sentence first tells us about a situation in the past and then describes what was happening inside that scene at that moment.
The past tense opens the scene and shows that everything happened earlier.
The present tense is then used to show the action that people were seeing or hearing at that moment.
It makes the description more vivid, as if we are watching the scene together with them.
Дети смеялись, наблюдая, как воздушный змей танцует на ветру.
The children were laughing, watching as the kite dances in the wind.
The first part simply tells us a fact from the past - the children were laughing. Then the focus shifts to the scene they are watching. The kite dances - this is the action happening in front of their eyes at that moment.
It is also possible to say everything in the past tense:
Дети смеялись, наблюдая, как воздушный змей танцевал на ветру.
The children were laughing, watching as the kite danced in the wind.
This is also grammatically correct, but it feels a little different. "Danced" sounds like a normal report about the past. "Dances" makes the scene more vivid, as if we are seeing it right now.
Such switches are very common in Russian, especially when someone is watching or listening to something:
Мы стояли и слушали, как оркестр играет.
We were standing and listening to how the orchestra plays.
Он сидел у окна и смотрел, как дождь падает на улицу.
He was sitting by the window and watching how the rain falls onto the street.
Она наблюдала, как дети бегают по двору.
She was watching how children run around the yard.
Even though the whole situation belongs to the past, the action inside
