Learn Hardcore Chinese: wǒ gēn tā yìqǐ zài jiā kàn diànyǐng.

Both 跟 and 和 can mean “with/and” here, and both are natural: 我跟她一起… / 我和她一起…. 跟 is a bit more colloquial; 和 is neutral/slightly more formal. 与 is very formal/written and uncommon in everyday speech: 我与她一起… sounds bookish.

Is 一起 necessary after 跟她? What changes if I drop it?

It’s not strictly required. 我跟她在家看电影 already implies “with her.” Adding 一起 makes the “together” idea explicit and is very common and natural: 我跟她一起在家看电影. With some verbs (e.g., 跟她说 = “speak to her”), 一起 removes ambiguity, but with 看电影 there’s no confusion.

Can I change the order of 一起 and 在家?

Yes; several placements are natural:

我跟她一起在家看电影。
我跟她在家一起看电影。
我在家跟她一起看电影。 Keep 跟…一起 together; don’t split 一起 away from the verb phrase or stick it before the object alone.

Is 在 here a location marker or a progressive (“be -ing”)? How do I say “right now”?

Here 在 marks location: 在家 = “at home.” To mark “right now,” use progressive markers:

我在家看电影呢。
我在家正在看电影。
我正在看电影。 Sentence-final 呢 often signals an ongoing action.

Should there be a measure word before 电影?

Not when you mean the general activity “watching movies”: 看电影. Add a measure word when specifying number or a specific film:

看一部电影 (one movie; 部 is the standard MW for films)
看两场电影 (two showings/screenings; 场 counts occasions)
那部电影 (that movie)

Why is there no 了? How would I mark a completed action or past experience?

Without aspect markers, the sentence is neutral (habit, plan, or context-determined). For a completed event, add 了 after the verb/object:

Completed: 我跟她一起在家看了（一部）电影。 Sentence-final 了 can indicate a change of state: 我们在家看电影了 (now we’re watching/ended up watching). For “have done before,” use 过: 我跟她一起在家看过电影.

Could I just say 我们 instead of 我跟她?

Yes: 我们一起在家看电影 is very natural and concise. 我跟她 explicitly names both participants and can add a slight emphasis on “me and her.”

Where do time words go (e.g., “tonight,” “tomorrow”)?

Typical order is Subject + Time + (跟/和 + Person + 一起) + 在 + Place + Verb + Object:

我今天晚上跟她一起在家看电影。
今天晚上我们在家一起看电影。 Time can also go sentence-initial for emphasis.

What’s the difference between 在家 and 在家里?

Both mean “at home.” 在家 is the common, neutral choice; 在家里 adds a slight sense of “inside the home/indoors” or a touch of explicitness. In everyday speech, 在家 is more frequent.

Does 在家 mean “at my home” here? How do I say “at her place”?

With 我 as the subject, 在家 usually defaults to “at my home” (unless context says otherwise). To specify, use possessives:

在我家 = at my place
在她家 = at her place Example: 我跟她一起在她家看电影。

In speech, how do I know if it’s 她 (she) or 他 (he) since both are pronounced tā?

You rely on context. If you must clarify in speech, add a descriptor: 那个女生 (that girl), 女的 (female), a name/title, or say 她本人 (she herself). Writing distinguishes 她/他; speech does not.

Why is 一起 pronounced yìqǐ and not yīqǐ or yíqǐ? What’s the rule for the tone of 一?

Tone sandhi for 一 (yī):

Before a 4th tone, 一 changes to 2nd: yí (e.g., 一次 yí cì).
Before 1st/2nd/3rd tones, 一 changes to 4th: yì (e.g., 一起 yìqǐ, 一年 yìnián, 一杯 yìbēi).
When said alone or as an ordinal with 第 (e.g., 第一), it’s yī (first tone).

Why isn’t there a 是 (“to be”) like in English “I am at home watching a movie”?

Chinese doesn’t use 是 to link subjects to locations or to form progressives. 我在家看电影 is correct as-is. Use the 是…的 pattern only for focusing/emphasizing details of a (typically past) event: 我是在家跟她一起看电影的 (emphasizes “it was at home and with her”).

How do I negate this? What’s the difference between 不 and 没?

Habit/refusal/future: 不 — 我不跟她一起在家看电影 (I don’t/won’t).
Past occurrence: 没(有) — 我没跟她一起在家看电影 (I didn’t).
Not currently happening: 我没在家看电影，我在外面。

How do I turn this into a yes–no question?

吗 question: 你跟她一起在家看电影吗？
A-not-A: 你跟她一起在家看不看电影？
Tag-style confirmation: 你跟她一起在家看电影，是吗？

What’s the difference between 一起 and 在一起?

一起 is an adverb meaning “together (to do something)” and goes before the verb: 我们一起看电影.
在一起 means “to be together/at the same place (often ‘to be a couple’)”: 我们在一起 (= we’re together). Don’t replace 一起 with 在一起 before a verb in this sentence.