> Sanskrit is a very rich language and old language. One word may have
> multiple meaning.
> 
> Language evolve on change time to time. Vedas were at least written
> around 1500bce.
> 
> So how do we know that the translation we are reading and
> understanding is correct. And that's how people have understood the
> Vedas when they were first composed.

That is why the Rishis have created four *[Vedangas][1]*: Vyakarana, Nirukta, Shiksha, and Chandas. 

 - Vyakarana is Vedic grammar and linguistics.
 - Nirukta is etymology of Vedic words.
 - Shikha is pronunciation of Vedic words.
 - Chandas is how Vedic poetic meters are constructed.

Each Vedanga has a set of texts written by celebrated Rishis.

Panini's *Ashtadhyayi* is a foundational treatise for understanding Vedic grammar.
Yaska's *Nighantu* for understanding Nirukta.
The *Pratishakhyas* for each Vedic shakha are to understand the Vedic shakhas.
*Chandas* and *Pingala* sutras to understand Chandas.

In addition to these Vedangas, there is the unbroken chain of Vedic recitation and transmission which is continuing even till today. There are [11 ways][2] to recite the Vedas:

> Eleven such ways of reciting the Vedas were designed – Samhita, Pada, Krama, Jata, Maalaa, Sikha, Rekha, Dhwaja, Danda, Rathaa, Ghana, of which Ghana is usually considered the most difficult.

Pada patha is word by word recitation.

On top of this we have the Smritis, which serve as a commentary on the Vedas:

> itihasa puranabhyam vedam samupabrahmhayet | bibhetyalpashrutadvedo namayam pratirishyati || - Mahabharata

> "One should interpret the Vedas with the Puranas and Itihasas"



  [1]: https://en.wikipedia.org/wiki/Vedanga#:~:text=The%20Vedanga%20(Sanskrit%3A%20%E0%A4%B5%E0%A5%87%E0%A4%A6%E0%A4%BE%E0%A4%99%E0%A5%8D%E0%A4%97%20ved%C4%81%E1%B9%85ga,)%3A%20phonetics%2C%20phonology%2C%20pronunciation.
  [2]: https://en.wikipedia.org/wiki/Vedic_chant#Pathas