As an English teacher, what do you start with when teaching grammar?

start with verb "to be" bec most of English grammar is built on it 

I usually start with the part of speech and proceed on to grammar and all language related skills.

1st ,  which grade would you want to explain to them  

It is normal to start with parts of speech. 

It depends on the level of the students, but I can say that I always teach grammar in context. Grammar is a tool for communication; I never teach it directly, I always try my best to guide the students and elicit the grammar point, after all you want the students to use it, not just memorize it.

I can start with a simple dialog/situation with a certain pattern/meaning and we build the rules from there.

To illustrate my point, let us think about how toddlers acquire language: they listen, understand, repeat, respond, try to communicate, and later are corrected. The child uses/experiences the language.

This is obviously an extreme example, but it holds some merit because the student feels more or less the same way when confronted with a new language. The teacher needs to have the student use/understand a certain pattern first and then teach/practice it.

      I think if we started by  parts of speech — verbs, nouns, pronouns, adverbs, adjectives, prepositions, conjunctions, and interjections will be better, in addition to that sentence formation(Subject-Verb-Object)  starting from simple sentence until we reach the Essay. 

I start when they have a decent knowlege of vocabulary, and spoke English. There is not an exact tine, each sudent is different. You will know.

It's not a black and white issue.

I would start with a warm-up activity wherein the target grammatical structure is used rather subconsciously. I would contextual my teaching.

I usually start with articles, present simple tense with verb to be and have.

I start with verbs, since it is the most necessary and influential part of the sentence, and then word order.