Mare (sea) is 3rd declension.
3rd Declension
Third declension.
First declension.
'Tempus' is third declension.
Third declension.
Consul, consulis is third declension.
Yes, declension is of a major importance
Esse is the verb "to be". Only nouns are declined, therefore it is in no declension.
The Latin noun Homo, hominis is in the third declension.
"Somnus" is a noun of the second declension in Latin. It is a masculine noun, meaning "sleep." The genitive form is "somni," which follows the typical pattern for second declension nouns.
The word declension is a noun. It is a grammatical term.
And from the first declension of the flesh I learnt man's tongue, to twist the shapes of thoughts Into the stony idiom of the brain. To understand spoken Latin clearly, you have to master the declension of its verbs.