Cometen videre possumus, dummodo nubes caelum non tegant.

Cometen is the accusative singular, so it is the direct object of videre: to see the comet.

The form looks unusual because this noun was borrowed from Greek, and Latin sometimes keeps a Greek-style accusative ending -en. So cometen is an acceptable form meaning the comet. In other Latin texts, you may also see a more fully Latinized form such as cometam.

It is built into possumus.

The ending -mus tells you the verb is first person plural, so possumus means we are able or we can. Latin often leaves out subject pronouns like nos unless they are needed for emphasis.

Because possum is followed by an infinitive.

So:

• possumus = we can / we are able
• videre = to see

Together, videre possumus means we can see or more literally we are able to see.

This is very normal Latin. English does something similar with be able to see, though with can English uses a bare verb instead of to see.

Dummodo means provided that, so long as, or as long as only.

It introduces a condition or limitation. In this sentence, it means something like:

We can see the comet, provided that...

It is a bit more specific than plain si (if). Dummodo often suggests on the condition that.

Because dummodo normally introduces a clause with the subjunctive.

So tegant is the present subjunctive, third person plural, of tegere (to cover).

English usually does not show this difference clearly, so we still translate it with an ordinary present: provided that the clouds do not cover the sky.

If Latin used tegunt, that would sound more like a straightforward statement of fact rather than a proviso.

Because of the verb.

Nubes can be a tricky form, since it can look singular or plural depending on context. But here the verb is tegant, which is plural. That tells us nubes must be nominative plural: clouds.

If the subject were singular, we would expect tegat: the cloud covers.

Because caelum is the direct object of tegant.

The clouds are doing the action, and the sky is what they cover. So Latin uses the accusative:

• nubes = the clouds
• caelum = the sky
• tegant = cover

So literally: provided that the clouds do not cover the sky.

Latin word order is much more flexible than English word order.

Putting Cometen first gives it a little prominence: it sets up what the sentence is about. English usually has to keep a stricter order, but Latin can move words around more freely because the endings show their grammatical role.

So Cometen videre possumus and Videre cometen possumus would both be understandable, but the given order puts extra focus on the comet.