"The grass is big."
Translation:घास बड़ी है।
For है, you use the ang ki matra when the subject is plural.
बिल्ली बड़ी है - The cat is big
बिल्लियाँ बड़ी हैं - The cats are big
We've used हैं in the second sentence because बिल्लियाँ is plural
सेब लाल है - The apple is red
सेब लाल हैं - The apples are red
Both the singular and plural forms of सेब are the same. But because of हैं in the second sentence, we can make out that we are talking about the plural 'apples'.
For the past tense form था, the masculine plural is थे. You only need the ang ki matra to distinguish the feminine plural form थीं from the feminine singular थी.