We use is in the present tense whereas we use was in the past tense. More specifically, we use is in present continuous tense as in the sentence ‘He is eating food. ‘ On the other hand,the verb was is used in past continuous tense as in the sentence ‘The bird was flying to its nest
The simple rule is that “is” is present tense and “was” is past tense. If it’s happening now, you use “is”. If it happened in the past, you use “was”. But yes, you quickly run into problems like the example you cite.
If you want to remember easily, you can think of was/were as the past tense form of the auxiliary verbs am, is and are. Generally, “was is used for singular objects and “were” is used for plural objects. So, you will use “was” with I, he, she and it while you will use “were” with you, we and they