今天。。。

我是入住。我入住海滩同男朋友。我們吃了牛排。

今天我和男朋友一起去了海滩，我们吃了牛排。OR 今天我和我男朋友一起入住了海滩附近的酒店，在那里我们一起吃了牛排。（the last one is batter)

今天，我跟男朋友去了海滩。我們吃了牛排。

You are from America,so your mother language is English ,right? According the grammar of English ,you don't use "am\is\are" in a sentence of which the structure is"subject+v(+object)",right?The predicate is the verb, so there shouldn't be "am/is/are" since there can only be one predicate in one sentence.If you use "am/is/are",there is no verb,this kind of the structure is "subject+be（copula)＋n./adj.（predicative)".You know these two structures are different and they can't be mixed up. Chinese is the same,入住is the verb,so there shouldn't be a 是。In English you say I check in instead of I am check in so here you say 我入住 instead of 我是入住。And there is also tenses in Chinese, for past tense you should add a 了。So the translation of "I checked in "or "I have checked in"is “我入住了”。Actually Chinese doesn't have much grammar differences between past tense and perfect tense.了is used in both.

今天。。。

我同男朋友一起入住海滩。我們吃了牛排。